Levin The Earth Through Time 10th c2013 txtbk

Continental Crust Appears Worldwide Origin of Precambrian “Basement” Rocks b 235

The continental crust is rich in feldspars, quartz, and metamorphism of tonalite, a variety of sodium-rich
muscovite. It is called felsic. The first oceanic crust diorite that contains at least 10% quartz.
formed about 4.5 billion years ago by partial melting of
rocks in the upper mantle (see Table 8-2). Oceanic Proof that Archean continents stood above sea
crust has less silica and more magnesium and iron than level is apparent in a 3.46-billion year old Australian
continental crust. It is called mafic. erosional unconformity that contains a fossil soil. The
unconformity and its associated ancient soil represent
Felsic crust started forming 100 million years later, the oldest land surface known, and they give clear
around 4.4 billion years ago. This occurred in sub- evidence of erosion and weathering on land in the
duction zones, where descending slabs of crust par- Archean.
tially melted. The less-dense components in the melt
slowly rose buoyantly to the surface, where they cooled Early Archean islands of continental crust proba-
to form continental crust. bly were small, perhaps less than 300 miles in dia-
meter. These initial patches may have grown larger by
Ancient patches of continental crust are represented mechanisms that continue to operate today. Two
by the 3.8-billion-year-old Amisoq Gneiss Complex tectonic plates bearing continental crust might have
of Greenland (Fig. 8-28) and Canada’s 4.04-billion- collided, merging their continental crust. Growth
year-old Acasta Gneiss. These old continental rocks might also have occurred as island arcs and micro-
are tonalite gneisses. They were formed by continents were added to the host continent along
subduction zones.

What did the early protocontinents look like? Geo-
logic evidence indicates that Archean protocontinents
were steep-sided, relatively small, narrow landmasses
with rugged shorelines. There is no evidence of the
wide continental shelves we have today.

As indicated by the distribution of rocks that are
3.0 billion to 2.5 billion years old, some continents had
grown large by late Archean time. In 2.6-billion-year-
old South African sediments, there is evidence of
Earth’s earliest glaciation. Prior to this time, Earth
may have been too warm for ice sheets to form.

FIGURE 8-28 The 3.8-billion-year-old Amitsoq Gneiss The Earliest Plate Tectonics
of Greenland. Gneiss is a foliated metamorphic rock. These
tonalite gneisses have a felsic composition, indicating that When did plate tectonics begin on Earth? Probably
they are continental crustal rocks. (M.S. Smith) not during the first several million years. Extreme heat
would have churned the mantle too rapidly for surface
materials to cool and form lithospheric plates.

However, in 3.8 billion-year-old rocks along the
southwestern region of Greenland we find the earliest
evidence of plate tectonics. The rocks containing this
evidence constitute the Isua supercrustal belt. (It is
called supercrustal because the rocks consist of sedi-
ments and volcanics that do not rest on their original
basement.) The Isua rocks include oceanic crust that
exhibits parallel tensional fractures filled with volcanic
rock (Fig. 8-29). As the fractures opened, they were
intruded by magma from the hot interior. The same
process operates today along the mid-ocean ridges.
Thus, the rocks of the Isua hold the evidence that plate
tectonics operated on Earth a mere 750 million years
after its formation.

We have other evidence of Archean plate tectonics:
Archean rocks in Canada and Scandinavia display
parallel bands of continental crust that have moved
against one another during plate collisions. We see
similar patterns of deformation in younger rocks
where continents have increased their expanse by add-
ing microcontinents along subduction zones.

236 c Chapter 8. The Earth’s Formative Stages and the Archean Eon

FIGURE 8-29 The dark layers of volcanic
rock in this outcrop of 3.8-billion-year-old
exposure of oceanic crust provide evidence
that plate tectonics started early in the
Archean. For scale, the boulder at top right
is about 5 m wide. (Courtesy of Maarten
DeWit)

Once plate tectonics was in operation, it supplied conglomerates, and, less frequently, by banded iron
the emerging continents with mantle-derived material formations.
that could be partially melted to form additions to the
continents. Archean tectonics would have been excep- Where did the greenstone and granulite associa-
tionally vigorous. Rates of mantle convection would tions form, and why do they occur in elongate belts?
have been far greater, thermal plumes more common, An early stage in the development of the greenstone-
midoceanic ridges longer and more numerous, and granulite association was the formation of volcanic
volcanism more extensive. arcs adjacent to subducting tectonic plates (Fig. 8-34).
Lavas and pyroclastics on the arcs were weathered and
GRANULITES AND GREENSTONES In general, Archean eroded to provide sediment to the adjacent trench.
rocks can be grouped into two major rock associations. The wet sediment and the ocean crust on which it
The first is the granulite association,which is composed
largely of gneisses derived from strongly heated and FIGURE 8-30 Exposure of greenstone showing well-
deformed granitic rocks. The second is the greenstone developed pillow structures, near Lake of the Woods,
association, composed of volcanic rocks along with Ontario, Canada. The original basalts were extruded
metamorphosed sediments derived from the weathering underwater during Late Archean time. During the
and erosion of the volcanics. Lavas of greenstone belts Pleistocene Ice Age, glaciers beveled the surface of the
exhibit pillow structures indicating they were extruded exposures and left behind the telltale scratches (glacial
under water (Fig. 8-30). striations). (Courtesy of Klaus J. Schulz)

Greenstones occur in troughlike or synclinal belts
(Fig. 8-31). They are prominent features of Archean
terranes of all continents. The Abitiba belt (Fig. 8-32)
of the Superior Province is the largest uninterrupted
greenstone belt of the Canadian Shield.

The most intriguing characteristic of greenstone
belts is the sequence of rock types, from ultramafic
near the bottom to felsic near the top (Fig. 8-33). In
many, the basal layers are komatiites. Next in the
sequence are basalts, which are somewhat less mafic.
Green minerals like chlorite in the metamorphosed
basalts impart the color for which greenstones are
named. Next are felsic volcanics (andesites and rhyo-
lites), which are overlain in turn by shales, graywackes,

Origin of Precambrian “Basement” Rocks b 237

FIGURE 8-31 Generalized cross-section through two greenstone belts. Note their
synclinal form and the sequence of rock types—from ultrabasic near the bottom to felsic near the
top. A late event in the history of the belt is the intrusion of granites. Ultramafic basal layers are
particularly characteristic of greenstone belts in Australia and South Africa but do not occur in
the Archean of Canada.

rested was carried downward in the subduction zone
and partially melted to form increasingly more felsic
magmas. The felsic magmas then worked their
way upward, engulfing earlier volcanics. Finally,
metamorphism accompanying compression altered
the rocks to form the granulites.

What steps were involved in the formation of
greenstone associations?

1. Ultramafic to mafic lavas ascended through rifts
in back-arc basins to form the lower layers of the
greenstone sequence of rocks.

2. Extrusion of increasingly more felsic lavas and
pyroclastics, as well as deposition of sediments,
derived from the erosion of adjacent uplands.

3. Compression of the back-arc basins to form the
synclinal structure characteristic of greenstone
belts.

4. Intrusion of the granite plutons that surround
the lower levels of greenstone belts.

FIGURE 8-32 Greenstone belts of the Superior Province. FIGURE 8-33 Stratigraphic sequence in a greenstone
(Thurston, P., 2002, Autochthonous Development of belt in Barberton Mountain Land, South Africa. (After
Superior Province Greenstone Belts?, Precambrian D.R. Lowe, 1980, Ann. Rev. Earth Planetary. Sciences,
Research, Vol 115, Issues 1–4, 11–36.) 8:145–167.)

238 c Chapter 8. The Earth’s Formative Stages and the Archean Eon

Back-arc basin sags by
Volcanic arc extension, opening conduits

Tonalite intrusions for extrusions of ultramatic

Subduction trench and mafic volcanoes

Sea level

Felsic Sediments
basement Felsic volcanoes
Mafic volcanics

Ultramafic volcanics

Asthenosphere
A

Late granite intrusions Greenstone belt

Metamorphosed
felsic rocks

B

FIGURE 8-34 A plate tectonic model for the development of a greenstone belt.
(A) The forward margin of an oceanic plate is subducted beneath a continental plate.
Subduction provides for the emplacement of wedges of oceanic crust and for the mixing and
melting that yields tonalite intrusions. Tonalite is a variety of the igneous rock diorite
containing at least 10% quartz. Behind the main-arc (closest to the subduction zone), is
the back-arc. It sags due to stretching, and the greenstone volcanic sequence is extruded.
(B) Compression forces the rocks in the back-arc into synclinal form and provides the pressure
and heat for metamorphism. Later, granites are intruded in and around the greenstone belt.

ARCHEAN SEDIMENTATION The sequence from ultra- bombardment from about 4.5 to 3.8 billion years
mafic or mafic to felsic rocks in greenstone belts records ago. The impacting bodies delivered huge amounts
the development of felsic crustal materials from older, of extraterrestrial organic materials, including amino
more mafic source rocks. The sedimentary rocks near acids—the building blocks of proteins. It is likely that
the top of the greenstone sequence provide clues to the some of these additions from outer space may have
nature of the earliest patches of Archean continental played an important role in the origin of life on our
crust. Layers of coarse conglomerates containing peb- planet, and other planets as well.
bles of granite indicate felsic source rocks. Graywackes
and dark shales in the greenstone sedimentary sequence Unfortunately, we have no unquestioned fossil evi-
bear evidence of deposition in deep-water environments dence for the progression from nonliving molecules to
adjacent to mountainous coastlines. living organisms. Our hypotheses for the origin of life
are based on present-day biology (again, the present is
In the greenstone sedimentary association, we do key to the past), coupled with evidence from Archean
not find blankets of well-sorted, ripple-marked, cross- rocks, meteorites, and the compositions of planets.
bedded sandstones. The existence of broad lowlands
that could be flooded by shallow interior seas is there- How Molecules Might Combine to Start Life
fore unlikely. Nor is there any evidence for wide
marginal shelf environments. For the development of life, let us use an analogy: In
the nonliving world, neutrons, protons, and electrons
cTHE ORIGIN OF LIFE combine to form atoms. Atoms in turn combine to
The splendid diversity of animals and plants on Earth form molecules. Molecules join to form complex
today is a consequence of billions of years of chemical structures. Perhaps in a similar fashion, the atoms of
and biologic evolution that began during the elements essential to organisms (carbon, oxygen,
Precambrian. As described earlier, the basic materials hydrogen, nitrogen, sulfur, and phosphorus) combine
from which microbial organisms could have been to form organic molecules. At some time in history,
assembled initially may have been carried to Earth these simple molecules might have combined or added
during the episode of heavy meteor and asteroid components to transform them into the complex
organic molecules essential to all living things.

Another analogy might be how small molecules of The Origin of Life b 239
nitrogen, sugar, and phosphorus compounds combine
to form large molecules of DNA, deoxyribonucleic Then, how did the amino acids aggregate to form
acid (recall Figure 6-15). Some researchers suggest proteins? Laboratory experiments suggest that the first
that such large, complex molecules might have orga- proteins could have formed on the surfaces of clay
nized themselves into organelles, bodies capable of particles. Clay minerals have a layered structure like
performing specific functions. Organelles might then that of the mineral mica. Clays include metallic ions
have combined to form larger entities that grew, that could concentrate organic molecules in an orderly
metabolized, reproduced, and mutated. array, causing them to align and link themselves into
protein-like chain structures.
Pulling Together the Pieces of Life
Simulating the Origin of Life
Life requires the elements carbon, hydrogen, oxygen,
nitrogen, phosphorus, and sulfur. Each is abundant in In 1953, Stanley Miller (1930–2007), an exobiologist
the Solar System today and undoubtedly was on (one who studies life beyond Earth), achieved the first
primitive Earth. What had to be accomplished was laboratory synthesis of amino acids. He simulated
to organize these elements into the systems of an Earth’s earliest atmosphere (methane, ammonia,
organism and to develop some mechanism for self- hydrogen, and water vapor) in a laboratory flask
replication. (Fig. 8-35). He then discharged sparks of electricity
(miniature lightning) into the mixture. The liquid
There are four essential components of life: yielded a bonanza of amino acids and other complex
organic compounds that enter into the composition of
1. Protein. Proteins are strings of comparatively all living things. In later experiments by others, similar
simple organic molecules called amino acids. Pro- organic compounds were produced from gases of
teins act as building materials and as catalysts that Earth’s pre-oxygen atmosphere (carbon dioxide, nitro-
assist in chemical reactions within the organism. gen, and water vapor), although in lower yields.

2. Nucleic acids. Nucleic acids are large, complex The main requirement for the success of these
molecules found in the nuclei of cells. The two experiments was the lack (or near absence) of free
classes of nucleic acids are RNA (ribonucleic acid) oxygen. To the experimenters, it seemed almost inevi-
and DNA (deoxyribonucleic acid). DNA can rep- table that amino acids would have developed in Earth’s
licate itself and carries the genetic code that deter- pre-life environment. Because amino acids are rela-
mines the way organisms develop and function. tively stable, they probably increased gradually to an
abundance that would promote their joining together
3. Organic phosphorus compounds. These into more complex molecules leading to proteins.
transform light energy or chemical fuel into
the energy required for cell activities. Researchers are now coming close to being able to
produce a living organism in the laboratory. Complete
4. A container. The cell membrane is an enclosing DNA of certain viruses have already been synthesized,
sack that keeps the cell’s components together so and an entire genome of a bacterium has been manu-
they can interact. It relatively isolates the cell’s factured by stitching together its chemical compo-
chemistry from outside interference and man- nents. One day soon, biologists may be able to
ages the interaction between the cell’s contents insert the synthetic DNA into a membrane-bound
and the world outside. vesicle in which replication would carry the protoge-
netic material into future generations. Mistakes during
Amino acids are the building blocks of proteins, replication would result in variation, and with that,
so they have an important role in developing larger natural selection would begin.
and more complex molecules. Two environmental
conditions early in Earth’s history may have provided Where Did Life Begin?
the spark for the natural synthesis of amino acids. First,
UV radiation bathed Earth’s surface. Experiments The idea of the sea as life’s birthplace has been with us
demonstrate that UV radiation can separate atoms a long time. Since Darwin’s days, biologists have
in water-ammonia-hydrocarbon mixtures and recom- spoken of the primordial ocean as a “rich organic
bine them into amino acids. Second, widespread light- broth” containing all the necessary raw materials for
ning in Earth’s early times could achieve the same life. However, this venerable hypothesis is now being
thing. Together or separately, UV and lightning may critically reexamined.
have stimulated the production of amino acids at
shallow depths in lakes or oceans. Another possibility As we have noted, there was very little oxygen in the
is that these important building blocks of life may have atmosphere prior to the advent of photosynthesis.
arrived on Earth from space. Amino acids have fre- However, there was some oxygen being generated
quently been identified in meteorites. by photochemical dissociation. (Recall that this is
a process in which ultraviolet light splits water

240 c Chapter 8. The Earth’s Formative Stages and the Archean Eon

Electrical
discharge

Vacuum Gases

Cooling jacket

Boiling FIGURE 8-35 An apparatus like this
water replicated the conditions of early Earth
and generated amino acids. An electric
Trap containing condensed spark in the upper right flask simulated
amino acids lightning. The gases present in the flask
reacted together, forming a number of
basic organic compounds.

molecules in the atmosphere, liberating oxygen.) Even Hyperthermophiles and Chemosynthesis
such small amounts of oxygen released by photo-
chemical dissociation would have destroyed the The evidence for such a stygian beginning of life is
compounds required for life. the presence of microbes called hyperthermophiles
(literally, high-heat lovers). These thrive in seawater
So if life didn’t originate in the warm surface waters exceeding 1007 C (water’s boiling point at sea level)
of the ocean, where did it begin? An answer comes and can live deep within fissures below the actual vents.
from scientists who cruised midoceanic ridges in div- They often are ejected in such great numbers that they
ing submersibles. Here they found evidence that life cloud the surrounding waters.
may have originated in total darkness, at great ocean
depths, among jets of scalding water rising from In the absence of light for photosynthesis, these
hydrothermal vents and volcanic chimneys (Fig. 8-36). organisms derive their energy from chemosynthesis.
Unlike photosynthesis, by which organisms synthesize
The midoceanic ridges are vast, offering a range of their own organic compounds by using carbon dioxide
temperatures. Their hot waters dissolve an abun- as a source of carbon atoms and sunlight as a source of
dance of elements vital for life, including phosphorus. energy, organisms employing chemosynthesis derive
Clays exist along the ridges, providing the templates their energy by oxidizing such inorganic substances as
needed to construct the organic molecules that pre- hydrogen sulfide or ammonia. This suggests that the
ceded life. first organisms were not phototrophs (organisms uti-
lizing photosynthesis), but chemotrophs. The chemo-
There are yet other places where life may have synthetic microbes around vents of the midoceanic
begun. Over the past decade, scientists have found ridges form the base of a food chain that supports an
clues that the first primitive cells may have originated astonishing variety of invertebrates. These include
deep underground, or in comets, or even within the shrimplike arthropods, crabs, clams, and tubeworms
Earth’s pre-oxygenic atmosphere. That atmosphere up to 10 feet long (Fig. 8-36C).
was rich in methane and molecular hydrogen. When
ultraviolet radiation from the Sun reacts with these How are hypothermophiles classified? They have
gases, amino acids may have been produced. It would DNA that is sufficiently different from that of bacteria
have been only a first step toward life, for there are to warrant placing them in a separate branch on the
many additional steps between the making of amino tree of life. In Chapter 6, we noted that all life can be
acids and the building of a living organism.

The Origin of Life b 241

A

FIGURE 8-36 Did life begin at vents along the midoceanic ridges? (A) Section across the central part of
a midoceanic ridge (spreading center) shows how hydrothermal vents occur on the ocean floor. Cool water
(blue arrows) is gradually heated as it descends toward the hot magma chamber. The heated water leaches
sulfur, iron, copper, zinc, and other metals from surrounding rocks and returns toward the surface (red
arrows). It discharges from hydrothermal springs or vents on the seafloor (small black plumes), where the load of
metallic elements supports a distinctive community of organisms. The hydrothermal vent in (B) is located
3100 m below sea level on the mid-Atlantic Ridge. Nutrient-rich vent water supports chemosynthetic
bacteria that ultimately support a variety of invertebrates, including giant tubeworms (C). (B) B. Murton/
Southampton Oceanography Centre/Photo Researchers, Inc., (C) # Ralph White/# Corbis)

organized into three domains, the Archaea, Bacteria, The tubeworms of hydrothermal vents are truly
and Eukarya. The hyperthermophilic and related remarkable creatures. They are able to grow more
microbes that feed on hydrogen and produce methane than 80 cm in a year, exceeding the growth rate of
belong in the Archaea. Other organisms, such as plants any other known invertebrate. As they have no mouth
and animals, whose cell structure includes a nucleus or digestive tract, they depend upon their symbiotic
and organelles, are in the Eukarya. relationship with internal thermophilic microbes for

242 c Chapter 8. The Earth’s Formative Stages and the Archean Eon

survival. (Symbiotic means that each has some life have originated at depth and moved upward as Earth’s
function that the other needs to survive. If either primordial surface environment became gradually more
the tubeworms or microbes disappeared, the other hospitable? Or both? Or neither? These questions are
would die out.) Spongy tissue inside the worm is challenging our ideas about how life originated on
loaded with Archaean organisms. As true chemo- Earth and about the probability of life on other planets.
trophs, these organisms oxidize hydrogen sulfide
into carbon compounds that nourish the tubeworm. There is yet another question about the origin of
life. Did it occur many times back in the Archean, or
You can envision the steps by which Earth’s first just once? Statistical tests involving proteins in orga-
hyperthermophiles may have originated. It began with nisms from all domains of life indicate that, although
the downward percolation of seawater through frac- life on Earth may have originated multiple times, only
tures and fissures in the seafloor. The water would one of those events produced the ancestor to all
reach the extremely hot rocks that supply lavas to the organisms living today.
midoceanic ridge volcanoes. Here the water would
heat to 10007 C, yet not boil due to the tremendous Feeding Life on Earth
confining pressure. This is like a car’s cooling system,
which pressurizes the hot “antifreeze” coolant to keep Every organism requires nutrients, the substances
it from boiling. present in food that can be used as a source of energy
for running the machinery to sustain the organism.
The superheated water would react with surround- How different types of organisms “eat” is itself a
ing rocks, extracting elements needed to construct history of how life on Earth developed. Each group
organic molecules. Such molecules would begin to evolved its own strategy for obtaining nutrition.
form as the hot waters gradually returned to the surface,
cooling as they made their way upward. During that Fermenters. Food gathered by ancestral hetero-
time, amino acids and other organic compounds trophs might have been externally digested by excreted
might be synthesized and combined to form the first enzymes before being converted to the energy
protocells. Growth, proliferation, and mutation might required for vital functions. In the absence of free
lead to primitive chemosynthetic microbes like those oxygen, there was only one way to accomplish this
seen today in hydrothermal vent environments. conversion—by fermentation. There are many varia-
tions of the fermentation process. The most familiar is
Life in Extremely Hostile Environments fermentation of sugar by yeast. Yeast organisms—
which are a type of fungus—disassemble organic mol-
Recent discoveries of organisms living in amazingly ecules, rearrange their parts, and derive energy for life
harsh environments provide a new view of how inevi- functions. A simple fermentation reaction is:
table and ubiquitous life on Earth is. The heat-loving
hyperthermophiles are an example. We also have Sugar þ yeast activity ! carbon dioxide
detected microscopic life near Earth’s frigid poles,
where temperatures dip to À113 C. Colonies of fila- þ alcohol þ energy
mentous (“stringy”) bacteria have been found building
stromatolites on the floors of ice-covered freshwater Autotrophs manufacture their own food. Eventually,
lakes in Antarctica. Microscopic life occurs in hot, the original fermentation organisms would have con-
barren deserts. It also occurs in submicroscopic spaces sumed the organic compounds of their environment,
between grains of solid rock more than 3 kilometers creating a food shortage. This scarcity might have
(2 miles) beneath Earth’s surface, and in rocks having caused selective pressures for evolutionary change.
temperatures exceeding 110 C. One such microbe, At some point prior to depletion of the food supply,
found at a depth of 2.8 kilometers (1.7 miles) in the organisms evolved the ability to synthesize their nutri-
course of drilling for natural gas, was appropriately tional needs from simple inorganic substances. These
named Bacillus infernus, the “bacterium from hell.” were the first autotrophic organisms. Their evolution
These subterranean microbes have been dubbed proceeded in diverse directions. Sulfur bacteria man-
lithotrophs (“rock nourishment”). ufactured their food from carbon dioxide and hydro-
gen sulfide. Nitrifying bacteria used ammonia as a
Most lithotrophs live off energy derived from source of energy.
hydrogen, iron, magnesium, and sulfur. Some thrive
on rocks heated by radioactive decay. In the gold mines Photoautotrophs, which employ photosynthesis,
of South Africa, prodigious numbers of microbes live were more significant to Earth history. They evolved
at roughly 3 kilometers depth. Microbes have been the ability to use sunlight to power the dissociation of
discovered in uncontaminated granites 3600 meters carbon dioxide into carbon and free oxygen. The
(2.24 miles) below Earth’s surface in Sweden. carbon was combined with other elements to enable
organisms to grow, and the oxygen was released as a
Did the ancestors of these microbes originate at the waste gas. This waste gas prepared Earth’s surface
surface and work their way downward? Or could life environment for the next important step in the

evolution of early organisms. Simplified, here is the The Origin of Life b 243
reaction for photosynthesis:
Prokaryotes and Eukaryotes
Carbon dioxide þ water þ sunlight
Lack of fossil evidence means that we cannot date the
! sugar þ oxygen transition from prebiotic evolution of molecules to
the organic evolution of life forms. We can only say
Heterotrophs (troph ¼ to nourish) can’t make their that the oldest fossils are prokaryotes.
own food, so they scavenge nutrients in their environ-
ment. Organisms that do so include animals, such as Prokaryotes. These organisms have genetic material
ourselves. The earliest heterotrophs probably con- (DNA) that is not packaged into a nucleus (Fig. 8-37A).
sumed small aggregates of organic molecules that Prokaryotes reproduce asexually, by cell division
were present in the surrounding water or cannibalized (Table 8-3). Cyanobacteria are prokaryotes that are
their developing contemporaries. capable of oxygenic photosynthesis. They first appear
in the fossil record late in the Archean. Prokaryotes are
Anaerobic organisms flourished during the time in the domains Archaea and Bacteria.
when Earth’s atmosphere lacked free oxygen. How-
ever, as photoautotrophs multiplied, billions upon Because prokaryotes are asexual, and simply divide
billions of tiny living oxygen generators began to to reproduce themselves, they are restricted in genetic
change the primeval oxygen-poor atmosphere to variability to whatever was in the original prokaryote.
today’s 21% oxygen atmosphere. Fortunately, the It is probably for this reason that prokaryotes have
change was gradual. Had oxygen accumulated too shown little evolutionary change through more than
rapidly, it would have been lethal to early oxygen- 2 billion years of Earth history. Nevertheless, such
intolerant microorganisms. Iron in rocks of the conti- organisms represent an important early step in the
nental crust readily bonded with the emerging oxygen, history of primordial life.
preventing too rapid a buildup of this chemically
aggressive gas. Eventually, organisms evolved oxygen- Eukaryotes. Evolution proceeded from the prokary-
mediating enzymes that permitted them to thrive in otes to eukaryotes. Their cells have a definite nucleus
the new atmosphere. and well-defined chromosomes (see Fig. 8-37). Unlike
prokaryotes, eukaryotes contain organelles—bodies that
Once the iron on Earth’s surface became saturated perform specialized functions. Examples are chloro-
with oxygen, it could no longer mediate the buildup. plasts (which convert sunlight into plant sugar) and
At that point, the gas began to accumulate in the mitochondria (which metabolize carbohydrates and
atmosphere and to dissolve in fresh water and sea- fatty acids to carbon dioxide and water, releasing
water. Solar radiation converted some oxygen (O2) to energy-rich phosphate compounds in the process).
ozone (O3), forming an absorptive shield against Evidence of eukaryotes extends to at least 2.2 billion
intense and harmful UV from the Sun. This protected years ago and possibly 2.7 billion years ago.
still-primitive and vulnerable life forms so they could
expand into new environments. Biologists believe that the organelles in eukaryotic
cells were once independent microorganisms that
Aerobic organisms. The stage was set for the moved into other cells and established symbiotic
appearance of aerobic organisms, which rely on oxy- (mutually beneficial) relationships with the primary
gen to live. Aerobic organisms are more efficient, cell. For example, one bacterium might engulf, but not
because using oxygen to convert food into energy digest, another bacterium. If the two survive together,
provides far more energy in proportion to food con- they reproduce and eventually might become organ-
sumed than anaerobic metabolism. This energy sur- elles (Fig. 8-38). The host cells would have the advan-
plus was critical to the evolution of more complex life tage of having an internal organelle capable of
forms. photosynthesis, whereas the other would benefit
from a protected environment and availability of
nutrients. Eventually, the two would lose their ability
to function outside the primary cell.

TABL E 8- 3 Comparison of Prokaryotic and Eukaryotic Organisms

Prokaryotes Eukaryotes

Organisms Bacteria and Archaea Protists, fungi, plants, and animals
Cell size 1 to 10 micrometersà 10 to 100 micrometers
DNA in chromosomes in membrane-bound nucleus
Genetic organization Loop of DNA in cytoplasm Membrane-bound organelles (chloroplasts and mitochondria)
Mitosis or meiosis, dominantly sexual
Organelles No membrane-bound organelles

Reproduction Binary fission, dominantly asexual

à A micrometer is 1=1000 of a millimeter.

244 c Chapter 8. The Earth’s Formative Stages and the Archean Eon

FIGURE 8-37 (A) Comparison of a prokaryote cell (left) and a
eukaryote cell. Note that the prokaryotic cell is tiny (0.5 to
1.0 micrometers). The eukaryotic cell is larger (10 to
100 micrometers), contains a true nucleus, and various
organelles. (B) A false color electron micrograph of the living
prokaryote Bacillus subtilus that is about to complete cell
division. Magnification Â335,000. (Photo Researchers, Inc.)

Eukaryotes reproduce sexually, with a union of egg not individual one-celled remains, but large sedimen-
and sperm to form the nucleus of a single cell. This tary structures formed indirectly as a response to the
combines the parental chromosomes, which leads to liberation of carbon dioxide by matlike colonies of
many possible gene combinations in the next genera- photosynthetic cyanobacteria.
tion. With the development of sexually reproducing
eukaryotes, genetic variations could be passed from Examination of present-day stromatolites, such as
parent to offspring in a great variety of new combina- those at Shark Bay, Australia (Fig. 8-39), indicates that
tions. This development was truly momentous, for it they develop when fine particles of calcium carbonate
led to a dramatic increase in the rate of evolution and settle on the sticky surfaces of cyanobacterial mats to
was ultimately responsible for the evolution of com- form thin layers (laminae). The bacterial colonies then
plex multicellular animals. grow through the layer and form another surface for
the collection of more fine sediment. Repetition of this
Archean Fossils process produces the succession of laminae. In many
Archean stromatolites, the calcium carbonate has been
Prokaryotic organisms existed during the Archean. replaced by chert.
Fossilized bacteria have been found in rocks dated
to about 3.5 billion years. Some geochemical evidence Stromatolites are more common in Proterozoic
indicates that prokaryotic organisms may have been rocks than in Archean, perhaps reflecting the more
present as early as 3.8 billion years ago. However, far common warm shelf environments during the Protero-
more abundant in the Archean than these earliest zoic. However, laminated structures resembling stro-
traces of life, are stromatolites. Stromatolites are matolites are present in 3.5-billion-year-old Archean
rocks of western Australia and 3.0-billion-year-old
Archean rocks of southern Africa. Microscopic

The Origin of Life b 245

FIGURE 8-38 A theory for the origin of
eukaryotes. A prokaryotic organism
engulfs other prokaryotes, which
function well in the protected
environment inside the parenty cell.
Eventually, the engulfed cells become
organelles of a eukaryotic organism.

FIGURE 8-39 Present-day and ancient stromatolites. (A) Modern stromatolites
growing in the intertidal zone of Shark Bay, Australia. Colonial marine cyanobacteria
form these structures. Fine particles of calcium carbonate settle between the tiny
filaments of the matlike colonies and are bound with a mesh of organic matter.
Successive additional layers result in distinctive laminations. (B) Fossil stromatolites
from Precambrian rocks exposed in southern Africa. At top right, note the rock hammer
for scale. ((A) Jane Gould/Alamy, (B) J. William Schopf)

GEOLOGY OF NATIONAL PARKS AND MONUMENTS

Voyageurs National Park bordered by rocky shorelines and con- events. The first chapter is recorded in
tain about 900 islands. Visitors leave severely deformed, compressed schists
Voyageurs National Park takes its name their automobiles at entry stations and and gneisses of Precambrian age. All
from the colorful French-Canadian explore by boat. exposed bedrock in the park originated
trappers who traveled the lakes in during the Precambrian (Fig. B photo),
birchbark canoes. The park lies along Like a history book in which all and most formations are Archean. The
the Minnesota-Ontario border (Fig. A), chapters have been destroyed but the last chapter is revealed in erosional and
near the southern margin of the Cana- first and last, the park displays a record depositional features from glaciers.
dian Shield (see map). The park con- of very early and very recent geologic
tains more than 30 lakes, which are

FIGURE A Location map Voyageurs
National Park.

FIGURE B Voyageurs National Park.
Minnesota Exposure of Precambrian gneiss
along the shore of Lake Kabetogama.
(The field of view is 40 m.) (R. F. Dymek)

246

Archean rocks at Voyageurs western edge of the park probably Chapter 15, this gargantuan blanket
National Park include the granulite derive from hydrothermal solutions of ice covered most of northern North
and greenstone associations described rising from the batholith. The gold America—over 2 kilometers thick in
in this chapter. Pillow lava within the veins caused a short-lived gold rush places. As it advanced slowly south-
greenstone association indicates that in the 1890s. westward, it easily dislodged huge
much of the Archean volcanic activity chunks of bedrock and transported bil-
here occurred beneath the sea. Meta- The latest Archean activity here was lions of tons of debris.
morphosed graywackes suggest rapid erosion of the mountain ranges pro-
deposition in trenchlike basins. duced by the Kenoran orogeny. Then, Today many large boulders from
about 2100 million years ago, black distant locations are scattered across
Voyageurs National Park lies in the basaltic dikes intruded into the older the Superior Province. These are gla-
Precambrian Superior Province. In Archean rocks. You can see the dark cial erratics. One erratic on Cranberry
the Late Archean, a mountain-build- rocks of the dikes along the shores of Lake’s south shore weighs over 200
ing event, the Kenoran Orogeny, many of the islands. tons. Rock fragments in the ice acted
created high temperatures that like crude sandpaper, scratching and
melted rocks at depth and emplaced If rocks younger than Precambrian grooving the underlying Precambrian
a granitic batholith. The pink and gray once covered Voyageurs National Park, rocks. When the ice receded, it left
granites of the batholith appear along it is likely that these rocks would have behind the glaciated landscape of lakes
the southern margin of Kabetogama been scoured and scraped away by and islands we know as Voyageurs
Lake. Gold veins in a fault zone at the the great continental ice sheet of the National Park.
Pleistocene Epoch. As you will see in

examination of these rocks reveal that they contain this discovery, eukaryotes were thought to have
filaments resembling bacteria, but the chemical attrib- appeared about 1.9 billion years ago. The evidence is
utes of these putative fossils have not been fully shown indirect—it does not consist of actual body fossils or
to contain matter of organic-origin. trace fossils, but of preserved organic molecules that
only eukaryotes can synthesize. These are called molec-
In addition to stromatolites, there are other ular fossils. They were carefully extracted from black
indirect indicators of cyanobacterial communities shales of Archean age in northwestern Australia.
that lived during the Archean. They are called by
the ponderous name, microbially induced sedimentary Although the molecular evidence indicates that
structures. They are structures in sediments that form eukaryotes apparently originated in the Archean,
as a result of cyanobacterial mats living on and in the they did not begin to diversify until about 1.2 billion
sediments. We see them in the 2.9-billion-year-old years ago, during the Proterozoic. Many believe the
Pongola sedimentary rocks of South Africa, and we see expansion of eukaryotic algae is related to the buildup
them as well in modern shallow marine environments. of oxygen in upper layers of the ocean, as well as the
The microbial mats and their associated filaments and production of nourishing quantities of nitrates.
adhesive films protect and bind sedimentary grains,
producing a variety of structures. They include polyg- cIN RETROSPECT
onal patterns in surfaces caused by shrinkage of the The Precambrian is the first act in the drama of Earth’s
mats during dry conditions, as well as randomly eventful history. During the Precambrian our planet
chaotic patterns of ripple marks. aggregated from a solar nebula; developed its internal
zones of core, mantle, and crust; achieved a magnetic
Molecular Fossils field; gave birth to continents; formed an ocean; and
wrapped itself in an atmosphere that became the one we
In 1999, Australian researchers found evidence of have today. It was a time when life appeared.
eukaryotic life on Earth 2.7 billion years ago. Prior to

SUMMARY

Earth is one of eight planets revolving around the Sun. of the core, mantle, and crust. The heat required for
Mercury, Venus, Earth, and Mars are the inner planets. differentiation resulted from radioactive decay,
They are built of rocky and metallic particles and have gravitational compression, and intense meteoritic
relatively high densities. Jupiter, Saturn, Uranus, and bombardment.
Neptune are the outer planets. They have lower densities
and are larger. As Earth was differentiating, gases were released (out-
gassed) from the interior. They accumulated as an initial
Earth formed from cosmic material in the solar nebula. It atmosphere devoid of free oxygen but rich in carbon
then underwent differentiation, resulting in the formation dioxide, water vapor, and other volcanic gases. An

247

248 c Chapter 8. The Earth’s Formative Stages and the Archean Eon

oxygen-rich atmosphere followed the origin and spread of Archean tectonics when granulite rocks formed along
oxygen-generating photosynthetic organisms. subduction zones adjacent to volcanic arcs and greenstone
belts developed in back-arc basins.
Archean rocks are most extensively exposed in broadly up-
warped, geologically stable Precambrian shields. Within During the Archean, life originated on Earth in an envi-
shields, there are Precambrian provinces recognized by ronment deficient in free oxygen and containing all the
their distinctive ages and boundaries marked by ancient elements needed to form complex, nonliving organic
orogenic belts. molecules. The earliest living cells to arise were hetero-
trophic prokaryotes. These were followed by autotrophic
The original crust of Earth was mafic and formed by prokaryotes, and eventually eukaryotes.
extrusions from the underlying mantle. From the original
mafic crust, patches of felsic crust formed by partial melting Life may have originated not near the surface of the
and recycling of the weathered products of the mafic crust. ocean, but at great depths along midoceanic ridges
where hydrothermal vents provided warmth and
Two main Archean rock complexes are the granulite and nutrients.
greenstone associations. They developed in the course of

accretion, p. 219 KEY TERMS
aerobic organism, p. 243
anaerobic organism, p. 243 magma ocean, p. 229
Archean Eon, p. 232 maria, p. 223
autotroph, p. 242 meterorite, p. 220
banded iron formation (BIF), p. 231 molecular fossils, p. 247
Canadian Shield, p. 233 nebular hypothesis, p. 219
carbonaceous chondrite, p. 220 ordinary chondrites, p. 220
chemosynthesis, p. 240 organelles, p. 239
chondrules, p. 220 outgassing, p. 230
craton, p. 233 partial melting, p. 228
differentiation (planetary), p. 228 photoautotroph, p. 232
eukaryote, p. 243 photochemical dissociation, p. 230
felsic, p. 235 photosynthesis, p. 230
fermenter, p. 242 platform, pp. 233
fusion, p. 219 plutoid, pp. 228
granulite, p. 236 Precambrian, p. 232
greenstone, p. 236 Precambrian provinces, p. 233
heterotroph, p. 243 prokaryote, p. 243
hydrologic cycle, p. 232 Proterozoic Eon, p. 232
hyperthermophile, p. 240 protoplanet, p. 219
iron meteorite, p. 220 shield (Precambrian), p. 233
komatiite, p. 229 solar nebula, p. 219
lithotroph, p. 242 Solar System, p. 216
lunar highlands, p. 223 solar wind, p. 219
mafic, p. 235 stony-iron meteorite, p. 220
stromatolite, p. 244
ultramafic, p. 236

Questions for Review and Discussion b 249

QUESTIONS FOR REVIEW AND DISCUSSION

1. At successively greater distances from the Sun, name the 15. Discuss the role of symbiosis in the evolution of
planets of our Solar System. In what galaxy are these planets eukaryotes. What organelles may have originated by
located? symbiosis?

2. Compare Mercury, Venus, Earth, and Mars. What do 16. What are hyperthermophiles? What does the presence
they have in common and how do they differ? of hyperthermophiles suggest about the possibility of finding
organisms on other planets in the Universe?
3. Given that both Earth and the Moon experienced heavy
meteoric bombardment during the Hadean, why are there so 17. How do the following organisms differ?
few craters recording that event on Earth?
a. Autotrophs and heterotrops
4. What would be the effect on Earth’s climate if the
planet’s orbit was strongly eliptical and not nearly b. Anaerobic organisms and aerobic organisms
circular?
c. Prokaryotes and eukaryotes
5. Why are we likely to learn more about the Hadean
history of Earth by studying the surface and rocks of the 18. Based on the age of the Earth’s oldest known rocks, as
Moon? well as the age of meteorites and samples of moon rocks
brought to the Earth, the age of our planet is about:
6. How did the internal layers of Earth develop?
___a. 2500 million years old
7. What are the principal kinds of meteorites? What is
the particular significance of carbonaceous chondrites? ___b. 542 million years old

8. Distinguish between the terms Precambrian shield, craton, ___c. 251 million years old
and platform.
___d. 6000 years old
9. Differentiate between the terms mafic and felsic. Give an
example of a felsic extrusive rock and one of a mafic extrusive ___e. 4560 million years old
rock. Name two minerals common to each of these rock
types. 19. In comparing eukaryotic organisms and prokaryotic
organisms, prokaryotes are:
10. How might patches of felsic continental crust have
been derived from mafic rocks during the Archean? ___a. Larger than eukaryotes

11. Describe the structural configuration and general ___b. Contain membrane-bound organelles
vertical sequence of rock types in a greenstone belt. In a plate
tectonics scenario, where might greenstone belts form and ___c. Undergo meiosis in reproduction
how would they develop?
___d. Have DNA within membrane-bound nucleus
12. How do komatiites differ from mafic rocks such as
basalt? Where do they occur in greenstone sequences? ___e. Lacking all of the above

13. What geologic evidence suggests that free oxygen was 20. Which of the statements below is not valid?
beginning to accumulate in Earth’s atmosphere about
2.5 billion years ago? ___a. Greenstone belt rocks occur in troughlike or
synclinal belts.
14. Why are Archean rocks more difficult to correlate
than rocks of the Paleozoic, Mesozoic, and Cenozoic? ___b. In a vertical sequence of greenstone belt rocks,
What is the method used to date and correlate most Archean there are ultramafic rocks at the bottom progress-
rocks? ing upward through mafic volcanics, then felsic
volcanics, and ultimately sedimentary rocks.

___c. Sedimentary rocks at the top of the greenstone
belt sequence display ripple marks and cross-
bedding indicating deposition in a shallow water
environment of deposition.

___d. The synclinal shape of greenstone belts resulted
from compression of back-arc basins.

9

The sculpture of four U.S. Presidents at Mount Rushmore
were carved from the Harney Peak Granite of Proterozoic
age. (Harold Levin)

CHAPTER 9

The Proterozoic: Dawn
of a More Modern World

We crack the rocks and make them ring, Key Chapter Concepts
And many a heavy pack we sling;
We run our lines and tie them in, In contrast to the Archean, the Proterozoic
We measure strata thick and thin, displays a more modern style of plate tectonics,
And Sunday work is never sin, sedimentation, and global climate.
By thought and dint of hammering.
The Wopmay Orogen, a belt of deformed rocks
—A. F. Lawson, Mente et Malleo (“Mind and Hammer”), 1888 in Canada’s Northwest Territories, developed
during a Wilson Cycle involving the opening
Lawson proved that Precambrian granites and gneisses of and closing of an ocean basin.
the Canadian Shield resulted from multiple mountain-
building episodes. Earth’s first great ice age occurred during the
Paleoproterozoic. In North America, tillites of the
OUTLINE Gowganda Formation attest to this frigid episode.
c HIGHLIGHTS OF THE PALEOPROTEROZOIC
Most of the world’s iron ore deposits occur in
(2.5 to 1.6 billion years ago) Paleoproterozoic banded iron formations (BIFs).
c BOX 9-1 ENRICHMENT: THE 18.2-HOUR The Animikie group in the Lake Superior region
and eastern Canada have yielded immense
PROTEROZOIC DAY amounts of iron ore.
c HIGHLIGHTS OF THE MESOPROTEROZOIC
During the Paleoproterozoic, Laurasia nearly
(1.6 to 1.0 billion years ago) broke apart. A rift zone (zone of tensional faults)
c BOX 9-2 ENRICHMENT: BIF: CIVILIZATION’S extended from the Lake Superior region to Kansas.
Magmas moved to the surface along the fault,
INDISPENSABLE TREASURE flowed onto Earth’s surface, and formed a thick
c HIGHLIGHTS OF THE NEOPROTEROZOIC sequence of basalt lava flows. The break failed,
however, and the continent remained whole.
(1.0 billion to 542 million years ago)
c PROTEROZOIC ROCKS SOUTH OF THE Cold climates return during the Neoproterozoic,
as indicated by tillites and glacial features on most
CANADIAN SHIELD of the world’s continents—even some that were
c BOX 9-3 ENRICHMENT: HELIOTROPIC located near the equator.

STROMATOLITES The Proterozoic Eon dawned 2.5 billion years ago
c PROTEROZOIC LIFE (Fig. 9-1) with a more modern style of plate tectonics,
c SUMMARY and ended only 542 million years ago. This enormous
c KEY TERMS periodcomprises42% ofEarthhistory.Toaidyourstudy
c QUESTIONS FOR REVIEW AND DISCUSSION of the Proterozoic, we divide the eon into three eras:

the early Paleoproterozoic Era (2.5 to 1.6 bil-
lion years ago)

the middle Mesoproterozoic Era (1.6 to 1.0 bil-
lion years ago)

the “new” Neoproterozoic Era (1.0 billion years
ago to the beginning of the Paleozoic Era,
542 million years ago)

251

252 c Chapter 9. The Proterozoic: Dawn of a More Modern World

Eonothem animals
Eon

Erathem
Era

System
Period
Ediacaran 542 m.y.a

Neo- 635 m.y.a
proterozoic Cryogenian 850 m.y.a
Tonian
Proterozoic 1000 m.y.a 542 m.y.a.

Meso- Ediacaran fauna
proterozoic
Supercontinent
1600 m.y.a Rodina begins
to break apart
Cryogenian
Paleo- glaciation
proterozoic
1000 m.y.a.
2500 m.y.a Abundant acritarchs

Rifting and massive Grenville orogeny
Keweenawan
lava flows

Eukaryotes

Maximum expansion
of stromatolites

1600 m.y.a.

Hudsonian orogeny closes
Paleoproterozoic in Canada

Gunflint Fossil
Prokaryotes

Formation of Wopmay
Orogenic belt

Multicellular algae Banded iron PROTEROZOIC
formations (BIF) STARTS HERE
Glaciation
2.2 b.y.a. 2500 m.y.a.PALEOPROTEROZOIC
Gowganda
glacial deposits *m.y.a. = millions of years ago

Kenoran orogeny
Closes Archean in Canada

FIGURE 9-1 Pathway through the Proterozoic, depicting major geologic events you will
encounter in this chapter.

Highlights of the Paleoproterozoic b 253

Because Proterozoic rocks are less altered than Proterozoic, continents became assembled into a new
Archean rocks, they are easier to interpret. However, supercontinent named Rodinia (see Fig. 9-10).
difficulties persist, for Proterozoic rocks lack the
abundant fossils of more modern Paleozoic- Another contrast between the Proterozoic and the
Mesozoic-Cenozoic strata. earlier Archean was major glaciation episodes. One
such “ice age” occurred during the Paleoproterozoic
A number of large, distinct crustal segments, called about 2.4–2.3 billion years ago. The second occurred
Precambrian provinces, developed in North America in the Neoproterozoic 850–600 million years ago.
during the Archean Eon. During the Paleoprotero-
zoic, these once-separated segments became sutured cHIGHLIGHTS OF THE
together to form Earth’s first known large continent, PALEOPROTEROZOIC
Laurentia. The suturing occurred along orogens— (2.5 to 1.6 billion years ago)
belts of crustal compression, mountain-building, and
metamorphism. By about 1.7 billion years ago, the The 900 million years of the Paleoproterozoic were
suturing was complete. dramatically eventful. Plate tectonics was in vigorous
operation. There was major mountain-building on all
For the remainder of the Proterozoic, Laurentia major continents. Earth experienced its first great ice
grew as more crustal materials became accreted to age, and there is geologic evidence of ample oxygen in
continental margins. These additions included the atmosphere.
sedimentary rocks crushed onto continental margins,
as well as microcontinents and island arcs carried to Early Plate Tectonics: Evidence from
subduction zones by seafloor spreading. Canada’s Northwest Territories

As the continents grew, so did tectonic plate During the Paleoproterozoic, orogenic belts devel-
motion, rifting, and seafloor spreading. On and around oped primarily around the margins of the older
the growing cratons, tectonic events and sedimenta- Archean provinces. One such Paleoproterozoic oro-
tion resembled those of more recent eras. By Protero- genic belt formed between 1.97 and 1.84 billion years
zoic time, wide continental shelves existed, as well as ago. It lies along the western margin of the Slave
shallow seas that flooded the continental interiors. province in Canada’s Northwest Territory (Fig. 9-2),
Such inland seas are termed epicontinental. Clean and has been named the Wopmay orogen.
sands and carbonate deposits accumulated. In contrast,
such sediments are rare in the Archean. Late in the

FIGURE 9-2 Rocks around the Wopmay Orogen in Canada’s Northwest Territories show
evidence of a Wilson Cycle. (A) Location on the west margin of the Precambrian Slave
province. (B) Cross-section of the Wopmay orogenic belt. It depicts a zone of intrusive igneous
rocks that pass eastward into metamorphic rocks and then into folded and thrustfaulted
stratified rock.

254 c Chapter 9. The Proterozoic: Dawn of a More Modern World

FIGURE 9-3 Relationship of rock

units following the

Paleoproterozoic opening of an

ocean basin along the western

margin of the Slave province.
Hoffman, P.F., 1989, Precambrian
geology and tectonic history of
North America, in Bally, A.W., and
Palmer, A.R., eds., Geology of
North America—An Overview:,
Boulder, Colorado, Geological
Society of America, Geology of
North America, vol. A, p. 447–512.

Here we see evidence for a Wilson Cycle, named upward into massive carbonates of the Rocknest
for J. Tuzo Wilson (1908–1993), a pioneer of plate Formation (Fig. 9-4).
tectonics theory. A Wilson Cycle includes three steps:
3. Closing of the ocean basin through plate
1. Opening of an ocean basin. Evidence for the tectonics. The leading edge of the Slave prov-
initial opening of an ocean basin consists of ince subducted beneath a microplate that
numerous normal (tensional) faults caused by approached from the west. The continental shelf
rifting, shown in Figure 9-3. Alluvial clastics buckled downward, forming a deep trough in
accumulated in the downfaulted blocks, and lavas which deep-water clastics were deposited. After
flowed upward through the fault planes to the trough filled, deltaic and fluvial sands were
become interlayered with sediments. deposited. All of these sedimentary layers were
later folded and faulted by plate collisions.
2. Sedimentation along the margins of separat-
ing continents. Continued rifting produced an The sequence of events in the Wopmay orogen closely
ocean basin that slowly widened. As it did so, parallels the Paleozoic evolution of the Appalachian
the western edge of the Slave province became Mountains. The history of both began with opening of
a passive shelf that received clastics from the an oceanic tract, progressed to the deposition of con-
coastal plain and shallow marine environment. tinental shelf and rise sediments along passive
Quartz sandstones (now quartzites) passed

FIGURE 9-4 Rocknest Formation. (A) Dolomite and interbedded shales of the Rocknest
Formation, Northwest Territories, Canada. The lighter colored layers are dolomite, and the
darker, rust-colored beds are shales. (B) Oblique aerial view illustrates dramatic folding due
to east-west crustal shortening and accretion during the orogenic phase of the development
of the Wopmay orogen. (Paul F. Hoffman)

Highlights of the Paleoproterozoic b 255

ENRICHMENT

The 18.2-Hour Proterozoic Day FIGURE B The changing length of the day through geologic time.

Earth has not always had its present 24-hour day. It has been daily growth increments. A coral might secrete one thin ridge
slowing at about 2 seconds per day each 100,000 years. The of calcium carbonate each day.
slowdown means that days have been increasing in length
through geologic time, and thus the number of days in the Wells also discerned coarse monthly bands, presumably
year has been decreasing. Why? related to breeding cycles when less calcium carbonate was
secreted. He also noted broader annual bands in corals that
Tidal friction causes the slowdown. As Earth rotates, lived in regions of seasonal change.
gravitational tug from the Moon and the Sun creates two
tidal bulges that travel around the planet daily, lifting ocean Wells counted the growth lines on several species of living
levels from a few centimeters to a few meters. However, the corals and found the count “hovers around 360 lines in the
Moon’s attraction prevents Earth from dragging the bulges space of a year’s growth.” Next, proceeding to fossil corals
very far. Thus, the two tidal crests tend to act as rather successively older in age, he counted correspondingly larger
inefficient brakes on either side of the rotating planet, numbers of growth lines in a yearly band (Fig. B). There were,
dragging back on the planet’s spin as it rotates. The bulges for example, 398 growth lines in the annual band of Devo-
also experience friction along shallower areas of the ocean nian corals. This means that the Devonian year had about 38
bottom from sea mounts, and are further retarded by con- more days than we have today.
tinents that stand in their way.
In another method for estimating the days in a paleo-year,
Earth’s slowing rotation was calculated by astronomers Charles Sonett used sedimentary rocks known as tidal rhyth-
decades ago. More tangible evidence came in the early mites. As seen along shorelines today, they are alternating
1960s from paleontologist John Wells, who recognized bands of dark and light silty deposits. Their thickness reflects
that fine lines (Fig. A) on coral exoskeletons might represent tidal extremes that mark the lunar month. Variations in the
thickness of sets of bands can be related to seasonal change.
FIGURE A Growth banding displayed by a specimen of the
extinct coral Heliophyllum halli. The finer Iines may represent Sonett’s analysis indicates that about 900 million years
daily growth increments. There are approximately 200 growth lines ago, during the Neoproterozoic, a day lasted only 18.2 hours.
per centimeter. Together with the annual bands, the growth The shorter days would have caused different weathering
increments can be used to estimate the number of days in rates, global temperatures, and weather patterns. We can
a year at the time the animal lived. (Harold Levin) only speculate about the effect of shorter periods of light and
heating upon life 900 million years ago.

continental margins, and ended with ocean closure and opening of an oceanic tract, deposition of sediment,
mountain-building. and subsequent closure along a subduction zone.
This closure, with its accompanying severe folding,
The Wopmay is just one of many orogens that metamorphism, and intrusive igneous activity, welded
developed along the margins of Archean provinces the Superior province to the Hearne and Wyoming
during the Proterozoic. Another is the Trans-Hudson provinces that lay to the west (see Fig. 8-28). Another
orogen, which extends southwestward from Hudson piece of continental crust was added to the North
Bay (Fig. 9-5). Rocks of the Trans-Hudson belt American craton.
record another Wilson Cycle of initial rifting,

256 c Chapter 9. The Proterozoic: Dawn of a More Modern World

FIGURE 9-6 Exposure of varved clays, Ontario, Canada.
The average thickness of the lighter colored layers is
about 2.5 cm. (Reproduced with the permission of Natural
Resources, Canada 2012, courtesy of the Geological Survey
of Canada.)

FIGURE 9-5 The Labrador Trough and Trans-Hudson alternate with tillites (unsorted, lithified glacial
orogen. (A) Location map. (B) Landsat NASA image of folded debris). This indicates periodic advances of ice into
Proterozoic rocks of the Labrador Trough. The belt was marginal lakes. Cobbles and boulders in the tillite were
once a chain of mountains, raised by the collision of two scratched and faceted by the abrasive action of an ice
continents about 1.8–1.7 billion years ago. Since then, the mass as it moved across the underlying bedrock. Some
mountains have eroded, exposing the deeper metamorphic and of the marine Gowganda sediments contain glacially
igneous rocks that were deformed into these folds by the scratched cobbles that were apparently dropped to the
collision. Note the lakes (dark blue) that reflect the trends of seafloor from melting icebergs.
the folds. Image taken from a height of about 900 kilometers.
((B) NASA) A basement of 2.6-billion-year-old crystalline rock
lies beneath the Gowganda Formation. The formation
itself is intruded by 2.1-billion-year-old igneous rocks.
Hence, the Paleoproterozoic glaciation occurred some
time between these two dates. Rocks similar to those of
the Gowganda are recognized in Western Australia,
Finland, southern Africa, and India.

Evidence of Earth’s First Ice Age? Iron, Oxygen, and BIFs

North of Lake Huron lies the Gowganda Formation, Paleoproterozoic rocks surround the western shores of
which includes conglomerates and laminated Lake Superior. They contain rich iron deposits that once
mudstones—clear evidence of glaciation. were the foundation of the Great Lakes steel industry in
Illinois, Indiana, Ohio, and Pennsylvania. The ore min-
Laminations in the mudstones represent alternating erals are oxides of iron, and thus they are evidence of
summer and winter layers of sediment called varves. free oxygen in Earth’s atmosphere. This implies that
Varved mudstones typically form in glacial meltwater photosynthesis, which is an oxygen-generating process,
lakes that are adjacent to ice sheets (Fig. 9-6). The probably was in vigorous operation by this time.
varved sediments in the Gowganda Formation

Highlights of the Mesoproterozoic b 257

FIGURE 9-7 Banded iron formation (BIF). This banded
iron formation from the Pilbara region of western Australia is
about 2 billion years old. (William D. Bachman/Photo
Researchers, Inc.) What mineral imparts the reddish color to
the darker bands?

Coarse sandstones and conglomerates deposi- occupy an adjacent zone to the east. That eastern zone
tedin shallow water lie near the base of the was subjected to intense folding (see Fig. 9-5B), meta-
Animikie Group. These rocks are overlain by cyclic morphism, and westward thrust faulting. The episode
successions of cherts, cherty limestones, shales, and of crustal deformation, called the Hudsonian orogeny,
banded iron formations (BIFs). BIFs are also known in marks the close of the Paleoproterozoic.
Archean sequences, but are not as extensive as those
of the Paleoproterozoic (Fig. 9-7). Some Canadian cHIGHLIGHTS OF THE
deposits are over 1000 meters thick and extend over MESOPROTEROZOIC
100 km. Within the banded iron sequence is the (1.6 to 1.0 billion years ago)
Gunflint Chert, a formation that contains an interest-
ing assemblage of cyanobacteria and other prokary- Highlights of the Mesoproterozoic include an aborted
otic organisms. ocean rift in the Lake Superior region, massive extru-
sions of basaltic lava flows, and the addition of copper
End of the Paleoproterozoic ores to the already iron-rich Lake Superior region.

East of the Superior province, along a curving, elongate An Aborted Rift, Rich in Copper
structural depression called the Labrador Trough
(see Fig. 9-5A), are Paleoproterozoic rocks. Like the Figure 9-8 shows a geologic cross-section from a
Wopmay orogen, these rocks were deposited on the point near Thunder Bay, Ontario (A) southeastward
continental shelf, slope, and rise. On the western side of across Lake Superior to the northwest shore of Lake
the trough, quartz sandstones, dolomites, and iron Michigan (A0). Study the cross-section to orient
formations lie above deposits in fault-bound basins. yourself. On the surface, note the little bit of Lake
Pillow lavas, basalts, mafic intrusives, and graywackes Michigan on the right, and how Lake Superior is

NW

Isle Royale L. Superior

L. Huron
SE FIGURE 9-8 Geologic

SW GraniteKeewaAtinnimkiIasnle RoyaleKeweenawan L. Michigan AnimikKiaeneAwnaitminwikiitahniron ores PaleSo2E01C cross section across
Lake Superior and
Michigan’s Upper
Peninsula.
(Adapted from U.S.

Geological Survey

Bulletin 1309 and Bedrock

Archean Igneous metamorphic Rocks of Michigan, small scale
map 2, 1968.)

258 c Chapter 9. The Proterozoic: Dawn of a More Modern World

ENRICHMENT

BIF: Civilization’s Indispensable Treasure original source of the iron. Volcanoes could have been a
source, or the iron may have been derived from deep weath-
Iron is the backbone of modern civilization, and much of the ering and erosion of iron-rich crustal rocks nearby.
iron we use comes from banded iron formations. These
formations were deposited in a relatively brief geologic At present, banded iron ores are being mined in North
interval between 2600 and 1800 million years ago. They America (in the Labrador Trough and the Lake Superior
occur in huge bodies that exceed hundreds of meters in region), the Ukraine, and in Western Australia. Substantial
thickness and thousands of meters in lateral extent. parts of the BIF are usable even as a low-grade ore called
taconite. Most of the high-grade BIF ores in North America
The most common type of BIF consists of bands of now are depleted, but taconite ore—in which iron ore is
hematite or magnetite alternating with lighter laminations concentrated into pellets—is now being shipped to iron
of chert. Most researchers believe that these deposits were smelters. The pellets contain about 60% iron. The most
chemically or biochemically precipitated. Fossils of bacteria important product made from iron is steel, which is an alloy
found in some of the deposits resemble present-day counter- of iron with a small amount of carbon to strengthen it.
parts that precipitate ferric iron hydroxide in oxygen-defi-
cient environments. But there is little agreement on the

interrupted by the land masses of Isle Royale and the zones associated with Keweenawan volcanism devel-
Keweenaw Peninsula. oped about 1.2 to 1.0 billion years ago and extended
from Lake Superior southward into Kansas. If these
Underlying it all are the crystalline basement rocks rifts had been extended to the edge of the craton, an
of the Archean and Paleoproterozoic Animikian. ocean tract would have formed within the rift system,
Above them, the Keweenawan rocks extend for hun- and the eastern United States would have drifted away.
dreds of kilometers. Keweenawan rocks consist of Rifting ceased, however, before such a separation
quartz sandstones, arkoses, conglomerates, and basal- could occur.
tic volcanics.
It is important to note that Lake Superior and the
The lava flows are noted for containing pure cop- other Great Lakes are not mini-oceans resulting from a
per. In the extruding lava, holes (vesicles) formed as gas failed continental rift! The Great Lakes, and the
bubbles, and these provided voids in which the copper thousands of lesser lakes and ponds in the area, result
was deposited. Copper also filled small joints and pore from glaciation. Vast ice sheets spread southward over
spaces in associated conglomerates. Long before Canada, gouging great basins that then filled with
Europeans arrived in the late 1600s, the Keweenawan glacial meltwater when the ice sheets melted back
copper deposits were mined by Native Americans. into Canada about 12,000 years ago. They have
From 1850 to 1950, copper production boomed in been lakes ever since.
this region, but in the mid-1970s, production ceased.
The Grenville Orogeny
The Keweenawan lavas accumulated to a thickness
of over 7 kilometers. The scene of the eruptions was a The Grenville Province of eastern North America was
hell on Earth, as a region the size of Indiana was the last to experience a major orogeny. Exposures of
engulfed in the searing heat of basaltic lava flows Grenville rocks extend from the Atlantic coast of Lab-
and steaming, acrid volcanic fumes. rador to Lake Huron. However, this is only part of their
true extent, for they continue beneath Phanerozoic
But even with so great an outpouring, the supply of rocks down the eastern side of the United States and
mafic magma was not exhausted. It remained beneath westward into Texas (see Fig. 8-28).
the surface, where it crystallized to form the long mass
of black, crystalline Duluth Gabbro—20 kilometers In the United States, interpretation of Grenville
thick and 250 kilometers wide. rocks is difficult, because they were heavily altered by
building of the Appalachian Mountains during the
We have learned that such great outpourings of lava Paleozoic. Originally, Grenville rocks were carbonates
are characteristic of seafloors. When this large quan- and sandstones (Fig. 9-9), but orogenic forces trans-
tity of mafic material comes to the surface within the formed them into metamorphic rocks containing
central stable region of a continent, it signals the many igneous intrusions.
presence of a rift zone, along which a continent may
break apart as the rift fills with ocean water. This is the Building a New Supercontinent—Rodinia
first stage of a Wilson Cycle. The tract where the break
begins typically develops tensional faults, along Deformation of Grenville sediments occurred 1.2 to 1.0
which mafic magma rises to the surface to form billion years ago during the Grenville orogeny. This
Keweenawan-like accumulations.

Evidence from gravity and magnetic surveys, as well
as samples from deep drill holes, indicates that the rift

Highlights of the Neoproterozoic b 259

About 750 million years ago, Rodinia began to
break apart. It was during this continental breakup
that the proto-Pacific Ocean—called Panthalassa—
formed. A great expanse of ocean now lay west of
North America. On the eastern side of North America,
Neoproterozoic sediments accumulated in basins and
shelf areas. These sandstones and shales were later
deformed during Paleozoic orogenic events. One such
event involved rifting and the filling of rift valleys with
lava flows and coarse clastics. Ocean water flooded the
rift valleys, forming a narrow seaway called Iapetus
that widened to form the proto-Atlantic Ocean.

cHIGHLIGHTS OF THE
NEOPROTEROZOIC
(1.0 to 542 million years ago)

FIGURE 9-9 Folded sandstones (dark) and carbonates of One of the most extraordinary events in the Earth’s
the Grenville Group. Belmont Township, Ontario, Canada. long history occurred during the Neoproterozoic.
(Reproduced with the permission of Natural Resources, Between about 850 and 635 million years ago, the
Canada 2012, courtesy of the Geological Survey of Canada.) surface of the planet became, either entirely or exten-
sively, covered with ice. Those that believe the planet
orogeny was part of the continental collisions involved was entirely enveloped in ice are proponents of the
in the formation of the supercontinent Rodinia (Fig. 9- controversial snowball Earth theory.
10). At the time of the Grenville orogeny, the east coast
of Laurentia (North America) lay adjacent to a block of Recognizing this astonishing episode of frigidity
western South America termed Amazonia. The west was the basis for naming the middle period of the
coast of Laurentia lay near Antarctica and Australia. Neoproterozoic, the Cryogenian (from the Greek cryos
meaning cold, and genesis meaning birth). The Cry-
ogenian Period was followed by the Ediacaran when
life on Earth seems to have exploded as the ice melted
away and warm climates returned. What geologic
evidence indicates a global ice age quickly followed
by global warming? And what could have triggered the
change from frigid to warm?

FIGURE 9-10 The supercontinent Rodinia as it appeared Glacial Rocks Beneath Tropical Rocks
about 1100 million years ago. The red band shows the
location of collisional orogens where continents collided in On every continent except Antarctica, geologists have
constructing the supercontinent. (Hoffman, 1991, Did the discovered Cryogenian sequences of coarse clastic strata
Breakout of Laurentia Turn Gondwanaland Inside-Out?, representing rock debris deposited either on land by
The American Association for the Advancement of Science.) glaciers or dropped to the ocean floor by melting ice
(so-called dropstones). These deposits, provided geo-
logists with the first evidence of worldwide Neoproter-
ozoic frigidity, even in tropical latitudes. Also, carbon
isotopes in the rocks suggest glaciation was accompa-
nied by low biological productivity, probably caused by
the global extent of cold conditions.

Above the Neoproterozoic glacial deposits, there is
a sharp transition to chemically precipitated dolomites
and limestones. These sedimentary rocks have been
dubbed “cap carbonates.” They indicate a sudden shift
to the warm conditions under which carbonates are
characteristically deposited. Possibly, a greenhouse
effect had been produced by a buildup of carbon
dioxide in the atmosphere from volcanic emissions.
Whatever the cause, global warming occurred, and
very likely promoted the explosion of life that charac-
terized the Ediacaran Period of the Neoproterozoic.

260 c Chapter 9. The Proterozoic: Dawn of a More Modern World Cryogenian
glaciation
What might have been the triggering mechanism
for the onslaught of a snowball Earth? Initial cooling
might have resulted from the eruption of one or more
super volcanoes, or a reduction in atmospheric gases
such as methane or carbon dioxide. Once ice began to
extend around the globe, the highly reflective icy
surface would increase the planet’s albedo, providing
a positive feedback for further cooling. It is note-
worthy that at this time, landmasses were distributed
along the equator. This would have allowed ice to
accumulate on land in latitudes where solar radiation is
most direct. As for the subsequent warming, theorists
suggest that carbon dioxide added to the atmosphere
from volcanoes over a span of millions of years would
suffice to melt the snow and ice. The blanket of ice
itself may have contributed to the preservation of
atmospheric carbon dioxide by inhibited photo-
synthetic uptake of the greenhouse gas by stromato-
lites and other microbial plants. Thus, we have the
paradox of glaciers causing their own destruction.

The debate over the validity of the snowball Earth
theory is likely to continue for many years. Whether or
not the planet was completely enveloped in ice remains
controversial. However, there is little argument about
the evidence for glaciation on a vast global scale.
Perhaps, as some paleoclimatologists suggest, ice
did not blanket the entire planet. A “slushball Earth”
in which ice did not extend beyond middle latitudes
seems to be gathering favor.

Earth’s Glacial History FIGURE 9-11 Earth has seen several major episodes of
widespread continental glaciation (orange). Each major
Figure 9-11 shows five major glacial episodes in Earth’s episode included intervals of glacial advance and retreat.
history. As described earlier, the Gowganda tillites indi-
cate widespread glaciation slightly more than 2 billion crushed rocks in southern Wyoming and western
years ago. The Neoproterozoic glaciations occurred Colorado—as shown by the red lines in Figure 9-12.
850–635 million years ago. Then, glacial ice advanced
again over major portions of the continents during the Following this orogenic event, there was extensive
Ordovician and Silurian, during the Carboniferous- Mesoproterozoic magmatism. Magmas that were
Permian, and again very recently, during the Pleistocene 1.5–1.4 billion years old intruded across a broad
Epoch. Each of these major glaciations included shorter belt of North America from California to Labrador.
intervals during which the ice sheets alternately grew
(colder climate) and shrank (warmer climate). The next event south of the Canadian shield was
widespread rifting. Large fault-controlled depressions
cPROTEROZOIC ROCKS SOUTH formed and filled with thick sequences of Neoproter-
OF THE CANADIAN SHIELD ozoic shales, siltstones, quartzites, and dolomites of

Precambrian outcrops occur south and west of the
Canadian Shield, although they are not as extensive as
in Canada. Thick sections are exposed in the Rocky
Mountains and Colorado Plateau. These rocks have a
complex history that began more than 2.5 billion
years ago.

First, an Archean terrane developed, composed of
strongly deformed and metamorphosed granitic rocks.
Next this old Archean mass collided with one or more
island arcs about 1.7 or 1.8 billion years ago. The line
of collision is marked by fault zones with severely

Proterozoic Rocks South of the Canadian Shield b 261

FIGURE 9-12 Shear zones (red) in Wyoming and FIGURE 9-14 Neoproterozoic stromatolites in
Colorado. These developed when the Archean craton collided carbonates of the Belt Supergroup, Glacier National
with island are terranes during the Paleoproterozoic. Green areas Park, Montana. (# Marli Bryant Miller)
are Lower Proterozoic outcrops. (Karlstrom and Bowring, 1988,
Early Proterozoic Assembly of Tectonostratigraphic Terranes in form towering cliffs in Waterton Lakes and Glacial
Southwestern North America, Journal of Geology.) National Parks (Fig. 9-13). Even though they are excep-
tionally thick, Belt Supergroup rocks display features
the Belt Supergroup. Exposures in Montana, Idaho, such as ripple marks and stromatolites (Fig. 9-14 ). These
and British Columbia reveal that the Belt Supergroup indicate that they were deposited in relatively shallow
is over 12 km thick. water along the passive western margin of North
America.
Because of their scenic features, Belt rocks are of
special interest. Massive carbonates of this unit

FIGURE 9-13 Chief Mountain near Glacier National Park, Montana. The upper third is
undeformed horizontal limestones of the Neoproterozoic Belt Supergroup. The lower two-thirds
are deformed Cretaceous beds. The “Lewis thrust” fault separates the two. It represents the
location where the Proterozoic beds were pushed eastward over weaker Cretaceous strata.

262 c Chapter 9. The Proterozoic: Dawn of a More Modern World

ENRICHMENT

Heliotropic Stromatolites Stromatolites are among the most abundant fossils in
Proterozoic rocks. The photosynthetic bacteria that con-
Heliotropic means “sun-turning.” A common example is the struct stromatolites depend on sunlight for survival and
sunflower, which tracks the Sun across the summer sky like growth. In a study of modern stromatolites, certain species
a dish antenna, aiming itself for best exposure to solar form columnar laminated growths that are inclined toward
energy. the Sun (heliotropic) to gather the maximum amount of light
on their upper surfaces.
FIGURE A 850-million-year-old stromatolites from the Bitter
Springs Formation of central Australia. Vertical dimension is 14 cm. Paleobiologist Stanley Awramic and astronomer James
(J. William Schopf) Vanyo have studied 850-million-year-old stromatolites from
Australia’s Bitter Springs Formation for evidence of heliotro-
pism (Fig. C), and believe they have found it. At the study
locality in central Australia, stromatolites curve upward in a
distinct sine-wave form. This form appears to have recorded
growth that followed the seasonal change in the position of
the Sun above the shallow sea in which the stromatolitic
cyanobacteria lived.

In their elongate sinuous form, each sine wave repre-
sented a year of growth. The stromatolites grew by the
addition of a layer or lamina each day. Thus, by counting
the laminae in the length of a single sine wave, one can
obtain the number of days in the year. The results indicate
that there were 435 days in a year 850 million years ago,
when the stromatolites of the Bitter Springs Formation were
actively responding to Proterozoic sunlight.

Grand Canyon Precambrian Rocks intensely folded and invaded by granites. These
granitic intrusions (and correlative rocks of the south-
Another area that features Precambrian rocks is the western United States) were emplaced 1.4–1.3 billion
Grand Canyon region. It has two distinct units, as you years ago as part of the Mazatzal orogeny.
can see in Figure 9-15. The lower, older unit is the
Vishnu Schist, and the upper unit is tilted strata that A splendid example of an unconformity occurs at the
we collectively call the Grand Canyon Supergroup. contact between the Vishnu Schist (see Fig. 9-15)
and the overlying Grand Canyon Supergroup. The
The Vishnu Schist (Fig. 9-16) is a complex body of latter unit is Neoproterozoic in age and is itself
metamorphosed sediments and gneisses that have been

FIGURE 9-15 Vishnu Schist,
Grand Canyon Supergroup, and
other rocks in the Grand Canyon
of the Colorado River. Indicate by
arrows and labels two kinds of
unconformities in this cross-section. Is the
conglomerate at the base of the Grand
Canyon Group an expected lithology for
the initial strata above an erosional
unconformity?

Proterozoic Life b 263

FIGURE 9-16 The Vishnu Schist exposed in the
walls of the Grand Canyon of the Colorado
River. The Vishnu was originally deposited as
sediment and was metamorphosed about 1700
million years ago. (From: “The Grand Age of Rocks:
The Numeric Ages for Rocks Exposed within
Grand Canyon,” by Allyson Mathis and Carl
Bowman (2006), published by The National Park
Service, U.S. Department of the Interior.)

uncomformably overlain by Paleozoic rocks. The deficient in oxygen, and these included the thermo-
Grand Canyon Supergroup correlates with the Belt philes of deep-sea hydrothermal springs.
Supergroup to the north. It consists mostly of clastic
rocks (sandstones, siltstones, and shales) that accumu- Stromatolites, which were relatively sparse during
lated in a troughlike basin that extended into the the Archean, proliferated worldwide during the
craton. The Chuar Group of the Grand Canyon Proterozoic (Fig. 9-18), but declined markedly by
Supergroup contains small, circular carbonaceous the end of the era. Today stromatolites are rare,
structures believed to be fossil algal spheres. primarily because the microorganisms that build

cPROTEROZOIC LIFE
Life at the beginning of the Proterozoic was not
significantly different from that of the preceding late
Archean. Blue-green scums of photosynthetic cyano-
bacteria (Fig. 9-17) constructed filamentous algal mats
around the ocean margins. Myriad prokaryotes floated
in the well-illuminated surface waters of seas and lakes.
Anaerobic prokaryotes multiplied in environments

FIGURE 9-17 Paleoproterozoic colony of the FIGURE 9-18 Two-billion-year-old columnar
cyanobacterium Eoentophysalis. The colony appears in a stromatolites from the Kona Dolomite near Marquette,
thin section of a Belcher Group stromatolitic chert, Belcher Michigan.Notethe rockhammerfor scale.(J.WilliamSchopf)
Islands, Canada. (H. J. Hofman)

264 c Chapter 9. The Proterozoic: Dawn of a More Modern World

FIGURE 9-19 Fossil prokaryotes from the
Gunflint Chert. The three specimens across
the top with umbrella-like crowns are Kakabekia
umbellata. The three subspherical fossils are
species of Huroniospora. The filamentous
microorganisms with cells separated by septa are
species of Gunflintia. (Courtesy of J. William
Schopf & Elso S. Barghoorn.)

them are eaten by marine snails and other grazing
invertebrates. The decline of Proterozoic stromato-
lites may be similarly associated with overgrazing by
newly evolving groups of marine invertebrates. Even
today, stromatolites survive only in environments
unsuitable for most grazing invertebrates.

Molecular fossils of eukaryotes first appear near the
end of the Archean, about 2.7 billion years ago.
Although prokaryotes still dominated during the Mes-
oproterozoic, eukaryotes became more abundant in
the fossil record. The latter part of the Neoproterozoic
witnessed the evolution of Earth’s first multicellular
animals (metazoans).

Microfossils of the Gunflint Chert FIGURE 9-20 (A) Eoastrion, (B) Eosphaera,
(C) Animikiea, and (D) Kakabekia from the Gunflint
The prokaryotes, which appeared in the Archean, Chert. All specimens are drawn to the same scale. Eosphaera
continued to thrive during the Proterozoic. We find is about 30 microns in diameter. (Barghoorn, E., 1971. “The
their fossils in stromatolites and former muds of Oldest Fossils,” Scientific American 224: 30–42)
coastal lagoons and mudflats. Along the northwestern
shore of Lake Superior are exposures of the Gunflint
Chert, a rock unit 1.9 billion years old that bears these
fossils.

You can see various stringlike filaments and ball-
shaped cells in thin sections of the chert (Figs. 9-19 and
9-20). Unbranched stringlike forms, some of which are
septate, have been given the name Gunflintia. More

finely septate forms, such as Animikiea, closely resem- Proterozoic Life b 265
ble certain living algae. Other Gunflint fossils, such as
Eoastrion (literally, the “dawn star”), resemble living FIGURE 9-21 A spinose acritarch. Acritarchs first appear in
iron- and magnesium-reducing bacteria. Kakabekia Proterozoic rocks. They are thought to represent the resting
and Eosphaera (the “dawn sphere”) do not resemble stage in the reproductive cycle of eukaryotic algae. The
any living microorganism, and their classification is organic coverings are resistant to chemical attack and can be
uncertain. These delicate fossils owe their preservation extracted from sedimentary rock by dissolving the rock in
to the glassy chert matrix in which they were hermeti- acid. Acritarchs probably represent the remains of primary
cally sealed, preventing destruction by oxidation. producers in the Proterozoic and Early Paleozoic seas. The
specimen shown here is about 28 microns in diameter.
The Gunflint fossils indicate that photosynthetic
organisms were abundant in the Paleoproterozoic. walls may be smooth or variously ornamented with
They were actively producing oxygen and thereby spines, ridges, or papillae (Fig. 9-21). Although their
altering the composition of the atmosphere. Not precise nature is uncertain, acritarchs appear to be
only do many of the Gunflint fossils resemble living phytoplankton that grew thick coverings during a
photosynthetic organisms, but their host rock contains resting stage in their life cycle. Some resemble the
organic compounds regarded as the breakdown prod- resting stage of modern marine algae known as dino-
ucts of chlorophyll. flagellates. (Dinoflagellates are among the organisms
that cause “red tides” that periodically poison fish and
In 1953 when these fossils were found, microscopic other marine animals.) Membrane-bounded nuclei can
fossils were rare and ambiguous. The well-preserved, be detected within some acritarchs, and their size is
abundant, and diverse Gunflint fossils alerted scien- comparable to that of living eukaryotes.
tists that microbial life was abundant about 2 billion
years ago and prompted the search for even older Acritarchs first appear in rocks about 1.6 billion
fossils. years old. They reached their maximum diversity and
abundance 850 million years ago and then steadily
The Rise of Eukaryotes declined. By about 675 million years ago, few
remained. Their decline coincided with the Varangian
The evolution of eukaryotes is a major event in the glaciation underway near the end of the Proterozoic.
history of life. Eukaryotes possess the potential for A reduction of carbon dioxide and an increase in
sexual reproduction, providing enormously greater atmospheric oxygen accompanying glacial conditions
possibilities for evolutionary change. Unfortunately, may have been responsible for the extinction of all
fossils of the earliest eukaryotes are rare. This is not but a few species that managed to survive until
surprising, considering that the first eukaryotes were Ordovician time.
microscopic cells whose identifying characteristics
(enclosed nucleus, organelles, and so on) are rarely In addition to acritarchs, protozoan eukaryotes
preserved. Size, however, is another important clue to were probably present in the Proterozoic. Protozoans
the identification of a fossil as a eukaryote. Living are nonphotosynthetic and derive their energy by
spherical prokaryotic cells rarely exceed 20 microns ingesting other cells (in other words, they are hetero-
in diameter, whereas eukaryotic cells are nearly always trophs). Foraminifera, amoebas, and ciliates are living
larger than 60 microns. examples of protozoans. Protozoan fossils are com-
mon in many Phanerozoic rocks, largely because they
Larger (and hence probably eukaryotic) cells begin evolved readily preservable shells.
to appear in the fossil record by about 2.7 to 2.2 billion
years ago. This earliest evidence of eukaryotic life In contrast, protozoan fossils from the Proterozoic
is based on biochemical remnants of eukaryotes. are much less common, because these organisms were
These chemical clues are termed molecular fossils. naked, shell-less creatures with little chance of preser-
Although eukaryotes evolved very early, they did not vation. The scarcity of Proterozoic protozoans,
begin to diversify until about 1.2 to 1.0 billion years
ago. It may be that they were unable to expand until
the oxygen content of the ocean reached a suitable
level, or perhaps they did not diversify until the advent
of sexual reproduction.

Acritarchs

Acritarchs (AK-ri-tarks) are a group of organisms that
are particularly useful in correlating Proterozoic
strata. They include diverse unicellular, spherical
microfossils with resistant single-layered walls. Their

266 c Chapter 9. The Proterozoic: Dawn of a More Modern World

FIGURE 9-22 Fossil metazoans from the Late
Proterozoic Conception Group, Avalon
Peninsula, Newfoundland. The fossils
(including circular impressions of presumed
jellyfish and elongate segmented animals) are seen
in the lower left quadrant of the photograph.
These early metazoans are similar to those of the
Ediacaran fauna of Australia. (James L. Amos/
Photo Researchers, Inc.)

however, does not mean that they were not abundant. Ediacaran organisms can be grouped into three
In fact, it is likely that the algal mats and masses of types, based on general appearance: discoidal (flat
decaying organic matter in late Proterozoic seas and circular), frondlike, and elongate. Discoidal forms
teemed with protozoans exploiting these rich sources such as Cyclomedusa (Fig. 9-23) are usually interpreted
of food. as jellyfish. Another discoidal form, Tribrachidium
(Fig. 9-24C), appears to have no modern counterpart
Many-Celled Animals Arrive: The Metazoans and may be a member of an extinct phylum.

The Archean fossil record consisted of relatively rare The Ediacaran frondlike fossils resemble living
remains of tiny, single-celled organisms. It was not corals informally called sea pens (Fig. 9-25). Sea
until the Proterozoic that fossil evidence of multi- pens look like fronds of ferns, except that tiny coral
cellular life is found. The most ancient multicellular polyps are aligned along the branchlets. The polyps
organisms are known from Paleoproterozoic shallow capture and consume microscopic organisms that float
marine rocks of Gabon, Africa. The record for multi- by. Frond fossils similar to those from Australia are
cellular life improves during the Neoproterozoic with also known from Africa, Russia, and England. In those
the discovery of metazoans preserved as impressions from England (Charniodiscus), the frond is attached to a
made in soft sediment before it hardened (Fig. 9-22). basal, concentrically ringed disk that apparently held
Metazoans are multicellular animals that possess the organisms to the seafloor. The disks are frequently
more than one kind of cell and have their cells orga-
nized into tissues and organs.

Ediacaran Biota FIGURE 9-23 Impression of a soft-bodied discoidal fossil
in the Ediacaran Rawnsley Quartzite in southern Australia.
The first important discovery of large metazoans in This organism has been interpreted as a jellyfish and named
Proterozoic rocks was made in Australia’s Ediacara Cyclomedusa. However, some paleontologists have concluded
hills in the 1940s. The Australian fossils and remains of that it is unrelated to any living organism. (B. N. Runnegar)
similar metazoans from other parts of the world have
been named the Ediacaran Biota. The oldest mem-
bers of this fauna have been discovered in China.

Although once thought to have disappeared by
Cambrian time, several animals within the fauna sur-
vived into the early Cambrian. For example, Ediacaran
fossils of Cambrian age have been reported in 510-
million-year-old rocks in Ireland. A few more possible
survivors of the Ediacaran fauna occur in the Burgess
Shale of the middle Cambrian age. This evidence
suggests that there was no mass extinction of
Ediacaran animals near the end of the Neoproterozoic,
as once proposed.

Proterozoic Life b 267

FIGURE 9-24 Three members of the Ediacaran fauna FIGURE 9-25 Diorama of the seafloor in which Ediacaran
from the Ediacaran Rawnsley Quartzite in southern metazoans lived. Prominent in this view are silvery jellyfish
Australia. (A) Pseudobizostomites, a wormlike form of and large, frondlike organisms interpreted here as soft corals
uncertain affinity. (B) Parvancorina, possibly an arthropod. known today as sea pens. (National Museum of Natural
(C) Tribrachidium, an unusual discoidal form that appears to History/Smithsonian Institution)
have no living relatives. Specimen (A) is an imprint in the
Rawnsley Quartzite; (B) and (C) are plaster molds made from found separated from the fronds, indicating that iso-
the original fossils. (Specimens by M. F. Glaesser; photos by lated discs once interpreted as jellyfish are actually the
Harold Levin) anchoring structures of frond fossils.

Is there an advantage in having a shape like that of
the frondlike fossils of the Ediacaran Biota? Most
shapes and structures in organisms serve a purpose.
In these fossils, the frond is supported by an erect stem
and stalk. Both serve to elevate the frond above the
seafloor. Thus, the organism could gather its food
particles and disperse its spores or gametes without
competition or loss from the activities of the many
animals populating the seafloor beneath the frond.

The third group of Ediacaran fossils are ovate
to elongate in form. The fossils are regarded as im-
pressions made by large flatworms and annelid
worms. Typical examples are Dickinsonia (Fig. 9-26),
which attained lengths of up to a meter, and Spriggina
(Fig. 9-27), a more slender animal with a distinctive
crescent-shaped structure at its anterior end.

Kimberella (Fig. 9-28) is a particularly significant
Ediacaran fossil. Four rather poorly preserved speci-
mens found at the Ediacaran site resembled jellyfish.
Then, in 1993, over 30 specimens of this unusual
organism were found on the shores of the White
Sea in northern Russia. Clearly, Kimberella was no
lowly jellyfish, but rather a complex invertebrate rank-
ing much higher on the evolutionary scale. Kimberella
has evidence of a true coelum, or body cavity in which
the digestive tract and other internal organs were
suspended. At 550 million years old, it is the earliest
animal known to possess this important characteristic
of all higher animals. It also is bilaterally symmetrical,
had a dorsal cover, and possesses a distinctive ruffled
border that some interpret as the edge of a mantle (the
organ that secretes the shell in mollusks).

268 c Chapter 9. The Proterozoic: Dawn of a More Modern World

FIGURE 9-27 Spriggina floundersi, interpreted by some
as a segmented worm from the Ediacaran Rawnsley
Quartzite of southern Australia. The animal is 3.5 cm
long. (Harold Levin)

FIGURE 9-26 Dickinsonia costata in the Ediacaran Seilacher argues that the resemblance between liv-
Rawnsley Quartzite of southern Australia. This fossil has ing sea pens and frond fossils is only superficial. He
been interpreted as a segmented worm. Scale in centimeters. notes that the branchlets in the frond fossils are fused
(B. N. Runnegar) together and do not have passages through which water
currents might pass. Living sea pens have such open-
Evidently, Kimberella crept across the substrate, ings, and this permits the polyps on the branchlets to
grazing on algae like some modern snails. Although catch food particles in the passing flow of water.
Kimberella appears very mollusk-like, there is no evi-
dence that it had a radula—the rasping tonguelike With regard to the discoidal fossils thought to be
structure found in most mollusks. But whether or jellyfish, Seilacher notes that living jellyfish have radial
not it is a true mollusk, it is important evidence that structures at their centers and concentric structures
advanced, complex invertebrates lived on Earth about around the periphery. This arrangement is opposite to
10 million years before the great “Cambrian explosion” that found in the discoidal Ediacaran fossils. Also,
of life. the rather superficial resemblance of Spriggina and
Dickinsonia to worms may have little significance, for
The Vendoza Controversy they exhibit no evidence of organs of modern-day
worms (such as a mouth, gut, and anus).
There is an interesting controversy about Ediacaran
fossils. Many paleontologists have interpreted them as Further, the soft, delicate tissue of Cnidarians and
Proterozoic members of existing phyla, such as the worms rarely is preserved in coarse sandy deposits. Yet
Cnidaria (which includes jellyfish and corals) and the Ediacaran animals have left distinct impressions in
Annelida (worms). However, this view has been ques- the sandstone. For this to occur, they must have had
tioned by paleontologist Adolf Seilacher. tough outer coverings. The ribbed and grooved
appearance of many Ediacaran impressions suggests
that these animals had an exterior construction like
that of an air mattress, providing the firmness needed
to make an impression in sand.

FIGURE 9-28 Reconstruction of
Kimberella. Kimberella is a
Proterozoic mollusk-like bilaterally
symmetrical organism. Specimens
range up to 10 cm in length. Name
an important characteristic of
Kimberella that indicates it is a highly
evolved multicellular animal.

The Ediacaran animals seem to explode suddenly Proterozoic Life b 269
upon the scene, presenting a remarkable range of body
types. However, that abrupt burst of evolutionary FIGURE 9-29 Cloudina, the earliest known calcium
variation quickly fizzled-out, providing an example carbonate shell-bearing fossils.Cloudina was first described
of sporadic evolution, described in Chapter 6 as punc- from the Proterozoic Nama Group of Namibia. It resembles
tuated equilbrium. a tube-dwelling annelid worm.
an opportunity for expansion of life into a global
Seilacher and his American colleague Mark environment that had become more hospitable. The
McMenamin postulate that Ediacaran animals lived day of the Ediacaran creatures would have arrived.
with symbiotic algae in their tissues, like modern
corals. This relationship would have given the animals Animals with Shells, and Those Known Only
a way to derive nutrition from the photosynthetic from Traces
activity of the algae. Also, the broad, thin shapes of The Ediacaran fauna tells us that Neoproterozoic seas
many of these animals might have given them suffi- were populated primarily by soft-bodied organisms.
cient surface area for diffusion respiration. This was However, some small shell-bearing fossils have also
essential to animals that had not yet evolved complex been found. One genus, first discovered in Neopro-
circulatory, digestive, and respiratory systems. terozoic rocks of Namibia, Africa, is named Cloudina
after the American geologist Preston Cloud (1912–
The dissimilarities of Ediacaran creatures and ani- 1991). Cloudina secreted a tubular, calcium carbonate
mals that exist today suggest that they do not belong in shell only a few centimeters long (Fig. 9-29). It has
existing phyla, but in a separate taxonomic category. been interpreted as a tube-dwelling annelid worm.
Seilacher proposed the name Vendoza (after Vendian, Other small shelly fossils that occur worldwide in
a term used in Russia for the final period of the sediments of latest Proterozoic and earliest Cambrian
Neoproterozoic). Seilacher’s Vendoza has since been include possible primitive mollusks, sponge spicules,
renamed the Vendobionta. and tiny tusk-shaped fossils called hyolithids.

Ediacaran Fossils and the Big Chill Not all Precambrian fossils are “body fossils.” Trace
fossils of burrowing metazoans exist in Neoproterozoic
After their sudden appearance about 630 million years rocks of Australia (Fig. 9-30), Russia, England, and
ago, Ediacaran organisms persisted with little change
for about 50 million years. We are left with the ques- FIGURE 9-30 Trace fossils made by a possible mollusk as
tion, “Why did they appear on the scene and expand so it crawled across soft seafloor sediment. The host rock
rapidly?” One explanation relates to the “snowball occurs at the Proterozoic–Cambrian boundary in British
Earth” hypothesis. Columbia, Canada. Originally, the traces had the form of
elongate depressions. Sediment deposited on top of the
As described earlier, there is abundant evidence of original layer then filled the depressions, so the crawling
global glaciation during the Neoproterozoic. Glacially traces are in convex relief. (H. J. Hofman)
produced scour marks and piles of lithified glacial
deposits are found on nearly every Neoproterozoic
landmass. Not only are these traces of glaciation found
in regions that were close to the poles, but also in
Neoproterozoic equatorial locations. Influenced by
this evidence, some geologists postulate that, during
the Neoproterozoic, ice covered not only regions of dry
land, but also the surface of the ocean. Earth had
become like a planetary snowball. If true, how might
this condition have affected life?

If the oceans were continuously covered by ice, this
would prevent ocean water from interacting with the
atmosphere. The ocean absorbs carbon dioxide and
oxygen from the atmosphere. Without a source of
atmospheric oxygen, levels of oxygen in the water below
the ice would plummet, causing severe extinctions of
marine organisms. This might account for the relatively
poor fossil record of life in pre-Ediacaran times.

Volcanoes, however, could have pierced the planet’s
icy envelope and discharged ample amounts of carbon
dioxide into the atmosphere. As carbon dioxide accu-
mulated, it could have initiated a greenhouse effect,
warming the planet and melting the ice. Here then was

270 c Chapter 9. The Proterozoic: Dawn of a More Modern World

North America. In every locality, they are in rocks Much of the oxygen became bonded to iron and other
deposited after the late Neoproterozoic Varangian gla- “oxygen-loving” elements. These elements served as
ciation. The traces consist mostly of relatively simple, “oxygen sinks” by capturing oxygen that otherwise
shallow burrows, whereas traces in the overlying Cam- would join the atmosphere.
brian are more complex, diverse, and numerous. There
is a similar increase in complexity, diversity, and abun- But about 2 billion years ago, the oxygen sinks became
dance of metazoan body fossils at the transition from the largely saturated, and free oxygen began to accumulate
Proterozoic into the Cambrian. in the atmosphere. Increased cyanobacterial photo-
synthetic activity probably contributed to this buildup.
Oxygen and Climate Changes in the (Figure 9-31 correlates this development with others of
Proterozoic Environment the time.) As atmospheric oxygen increased, so did
oxygen in the sea. It combined with nitrogen in seawater
Earth’s early atmosphere and hydrosphere were to form nitrate, an important nutrient for eukaryotic
largely devoid of free oxygen, for good reason: algae. This may help explain the expansion of acritarchs
and other eukaryotes during the Paleoproterozoic.

FIGURE 9-31 Correlation of major events in the biosphere, lithosphere, and atmosphere.

Another result of the oxygen buildup was the exten- Key Terms b 271
sive accumulation of ferric iron oxide, which stained
terrestrial sediments a rusty red. These red beds vali- where warm, tropical conditions prevailed, much
date the existence of an oxygenic environment. How- as they do today.
ever, the oxygen level probably increased slowly, and
likely did not approach 10% of present atmospheric Proterozoic evaporite deposits indicate warm
levels of free oxygen until the Cambrian Period. climates that were likely to have been arid as well.

In general, Proterozoic rocks provide evidence for a In the lower latitudes, climates were more severe,
wide range of climatic conditions. But we do not think as indicated by consolidated deposits of glacial
these climates were unique compared to more recent debris (tillites) and glacially striated basement
ones. For example: rocks. As previously discussed, the best known
of these poorly sorted, thick, boulderlike deposits
Thick limestones and dolomites with reeflike is the Paleoproterozoic Gowganda Formation.
algal colonies were deposited along the equator, The ice returned dramatically during the
Neoproterozoic, when continental glaciers cov-
ered extensive areas of the globe.

SUMMARY

By the beginning of the Proterozoic, many small cratonic South of the Canadian Shield, Proterozoic rocks are
(continental) elements that formed during the Archean exposed in the Grand Canyon of the Colorado River
had collided and had become sutured together to form and many localities in western North America. The Belt
large cratons. Supergroup is a prominent Proterozoic unit deposited in
the rifted western margin of North America.
North American Proterozoic orogens like the Wopmay,
Trans-Hudson, and Grenville indicate the full operation Life at the beginning of the Proterozoic resembled that of
of plate tectonics during the Proterozoic. the Late Archean. It consisted of stromatolites and various
stringlike and spherical prokaryotic microbes. By
The Neoproterozoic Grenville orogeny resulted from the Mesoproterozoic time, stromatolites and prokaryotes
suturing of Laurentia to other crustal segments during the were joined by eukaryotes.
assembly of the supercontinent Rodinia.
The most significant biological event of the Neoproter-
Widespread glaciations occurred during the Proterozoic. ozoic was the appearance of metazoans (multicellular
Particularly extensive episodes of glacial conditions animals).
existed during the Paleoproterozoic and Neoproterozoic.
Fossils of Neoproterozoic metazoans include large discoi-
Banded iron formations (BIFs) are an important source of dal forms, frondlike forms, and elongate forms tradition-
iron ore. They also provide important evidence of the ally considered to be members of presently known phyla.
buildup of sufficient atmospheric oxygen to oxidize iron at They may, however, belong to a new taxonomic group.
Earth’s surface.
Proterozoic climates ranged from warm tropical or sub-
The Mesoproterozoic Keweenawan basalts were extruded tropical, as suggested by extensive stromatolitic carbon-
along a rift zone extending from Lake Superior to Kansas. ates, to cold, as reflected in two episodes of continental
The rift zone failed to develop into an ocean opening that glaciation.
would have split North America into two continents.

acritarchs, 265 KEY TERMS
Animikie Group, p. 257
coelum, p. 267 Mesoproterozoic Era, p. 251
cyanobacteria, p. 257 molecular fossil, p. 262
dropstones, p. 259 Neoproterozoic Era, p. 251
Ediacaran Biota, p. 266 orogen, p. 253
epicontinental, p. 253 Paleoproterozoic Era, p. 251
Grenville orogeny, p. 259 Precambrian province, p. 253
Hudsonian orogeny, p. 257 red beds, p. 271
Keweenawan, p. 258 Rodinia, p. 253
Labrador Trough, p. 257 tillite, p. 256
Laurentia, p. 253 varves, p. 256
metazoan, p. 266 Vendobionta, p. 269
Vendoza, p. 269
Wilson Cycle, p. 254

272 c Chapter 9. The Proterozoic: Dawn of a More Modern World

QUESTIONS FOR REVIEW AND DISCUSSION

1. Describe the sequence of events recorded by the rocks of 13. Where was the Belt Supergroup deposited? What
the Wopmay region of northwestern Canada. evidence indicates that these rocks were deposited in shallow
coastal areas?
2. With regard to the history of Earth’s atmosphere, what
is the significance of banded iron formations (BIFs)? 14. Sedimentary features in the Paleoproterozoic Gowganda
Formation that indicate an episode of glaciation
3. When was the supercontinent Rodinia assembled? What
orogenic event in eastern North America was the result of the ___a. Tillites
assembly of Rodinia?
___b. Mudcracks
4. What kind of tectonic activity was the probable cause of
the massive outpourings of Keweenawan lavas in the Lake ___c. Graded bedding
Superior region? When did this occur?
___d. Cross-bedding
5. What features of eukaryotes are not present in
prokaryotes (see Chapter 6)? When do eukaryotes first ___e. Asymmetric ripple marks
appear in the fossil record?
15. Name given to a sequence of events that begins with
6. Paleontologist Andrew Knoll has stated that the opening of an oceanic tract followed by deposition of
“cyanobacteria are the heroes of Earth history.” sediment along a passive continental margin, and ending
Why do these lowly organisms deserve such praise? with compressive closure of the ocean tract

7. Were acritarchs eukaryotic organisms? When did ___a. Rock Cycle
acritarchs reach their maximum diversity, and when did
they nearly become extinct? What climatic conditions may ___b. Wilson Cycle
have contributed to their decline?
___c. Hydrologic Cycle
8. What are metazoans? What is the earliest known
occurrence of abundant metazoans? With regard to their ___d. Orogen Cycle
general appearance, what are the three major groups of
Ediacaran metazoans? 16. Name of the Neoproterozoic supercontinent that
existed from about 1100 million years ago until about
9. When rafting through the Inner Gorge of the 750 million years ago
Grand Canyon of the Colorado River, what Proterozoic
rock unit would you see exposed in the walls of the ___a. Panthalassa
gorge?
___b. Laurentia
10. When did continental glaciation occur during the
Proterozoic? What is the evidence that such glaciation ___c. Rodinia
occurred? Why is it unlikely that continental glaciers would
have formed during the earlier Archean? ___d. Laurentia

11. What characteristics of the Ediacaran discoidal fossils ___e. Shangri-La
suggest that they may not really be jellyfish, that the frond
fossils may not be sea pens (soft corals), and elongate forms 17. Which of the statements below can be considered
such as Dickinsonia may not be worms? incorrect based on the Grand Canyon rock sequence
depicted in Figure 9-15?
12. Stromatolites were exceptionally widespread
during the Proterozoic but became relatively sparse ___a. The Zoroaster Granite is the oldest rock unit.
thereafter. What other organisms may have contributed to
the post-Proterozoic decline of stromatolites by grazing ___b. A nonconformity exists at the top of the Vishnu
on them? Schist.

___c. An angular unconformity exists at the top of the
Grand Canyon Supergroup.

___d. The igneous sill was intruded after the deposi-
tion of the Bass Dolomite.

___e. The Tapeats Sandstone, overlain by the
Bright Angel Shale, in turn overlain by the Muav
Limestone is the typical sequence of rock types
representing deposition in a transgressive sea.



10

This vertical cliff of Cambrian sandstone (500–540
million years old) rises about 90 meters (300 feet) above
Lake Superior’s south shore, near the city of Munising in
Michigan’s Upper Peninsula. (R. F. Dymek)

CHAPTER 10

Early Paleozoic Events

If in late Cambrian time you had followed the present Key Chapter Concepts
route of Interstate 80, you would have crossed the
equator at Kearney, Nebraska. The early Paleozoic began 542 million years ago
(Fig. 10-1). At that time there were six major
—John McPhee, In Suspect Terrain, 1983 continents derived from the breakup of Rodinia:
Laurentia, Baltica, Kazakhstania, Siberia, China,
OUTLINE and Gondwana.
c DANCE OF THE CONTINENTS
c SOME REGIONS TRANQUIL, OTHERS ACTIVE The closing of the Iapetus Ocean resulted in the
c IDENTIFYING THE BASE OF THE CAMBRIAN Ordovician Taconic orogeny and the Devonian
c EARLY PALEOZOIC EVENTS Acadian orogeny. Erosion of the mountains
c CRATONIC SEQUENCES: THE SEAS COME IN, caused by these orogenies produced immense
wedge-shaped deposits of clastic sediments.
THE SEAS GO OUT The Queenston clastic wedge was formed from
c THE SAUK AND TIPPECANOE SEQUENCES the erosional detritus of mountains formed
c WAY OUT WEST: EVENTS IN THE during the Taconic orogeny.

CORDILLERA Early Paleozoic continents were composed of
c DEPOSITION IN THE FAR NORTH quiet stable interior regions and active orogenic
c BOX 10-1 GEOLOGY OF NATIONAL PARKS belts. Sands and carbonates deposited in shallow
epicontinental seas were the dominant sediments
AND MONUMENTS: JASPER NATIONAL PARK of the stable interior. Deep-water deposits
c DYNAMIC EVENTS IN THE EAST interspersed with volcanics characterized
c BOX 10-2 ENRICHMENT: A COLOSSAL marginal orogenic belts.

ORDOVICIAN ASH FALL Four major cycles of marine transgression and
c THE CALEDONIAN OROGENIC BELT regression dominate the Paleozoic history of the
c ASPECTS OF EARLY PALEOZOIC CLIMATE craton. For the early Paleozoic they are the Sauk
c BOX 10-3 ENRICHMENT: THE BIG FREEZE IN and Tippecanoe cratonic sequences, followed in
the late Paleozoic by the Kaskaskia and Absaroka
NORTH AFRICA cratonic sequences.
c SUMMARY
c KEY TERMS The Cordilleran side of North America began
c QUESTIONS FOR REVIEW AND DISCUSSION quietly as a passive margin, but by Ordovician
c WEB SITES time a subduction zone formed as the Pacific plate
moved against the continent’s western margin.

Rocks of the Phanerozoic Eon yield their secrets more
readily than Archean and Proterozoic rocks. They are
more accessible, less altered, and more fossiliferous.
The Phanerozoic Eon includes three eras: Paleozoic
(“ancient life”), Mesozoic (“middle life”), and Cenozoic
(“recent life”). We now focus on the geologic history of
the earliest one, the Paleozoic Era. This chapter looks
specifically at its oldest three geologic periods—the
Cambrian, Ordovician, and Silurian. These three peri-
ods together lasted about 126 million years.

In general, the geologic history of the Paleozoic
is characterized by long periods of tranquil
sedimentation, punctuated by intervals of convulsive
mountain-building. Just before the Paleozoic began,

275

276 c Chapter 10. Early Paleozoic Events

251*Paleozoic Era
Permian PeriodCarboniferous

299 Great Ordovician Biodiversity
Pennsylvanian Event (GOBE)
Period

318
Mississippian
Period

359
Devonian Period

416
Silurian Period

447
Ordovician Period

488
Cambrian Period

542

*Ages in millions
of years ago

FIGURE 10-1 Major events of the Paleozoic Era. (Courtesy R. F. Dymek)

the supercontinent Rodinia fragmented to form a num- Some Regions Tranquil, Others Active b 277
ber of smaller continents. These continents ultimately
would converge to form another supercontinent called At this time, the South Pole was located across from
Pangea. In North America, the mountain-building Laurentia in what is now central Chile. This explains
events are called the Taconic, Acadian, and Allegheny why late Neoproterozoic glacial conditions were wide-
orogenies. spread in Laurentia, as well as parts of Gondwana,
Baltica, and Siberia.
cDANCE OF THE CONTINENTS
What was the world like in the early Paleozoic? Where Laurentia had parted company with South America
were the continents and what events marked their by Cambrian time. The continent drifted northward
travels, separations, and collisions? To begin the story, until it lay astride the Equator (Fig. 10-3A). Climatic
we need to travel back to the late Neoproterozoic. The conditions changed from Neoproterozoic cold to
breakup of the Neoproterozoic supercontinent Rodi- Cambrian warm. An ocean tract named Iapetus
nia produced six large continents and several smaller opened east of Laurentia, so that the continent’s
microcontinents. The larger continents were: eastern side became a passive margin.

Laurentia, composed mainly of North America, By Ordovician time (Fig. 10-3B), Gondwana had
but including parts of Greenland, northwestern moved southward so that what is now North Africa was
Ireland, and Scotland. centered near the South Pole. Upper Ordovician till-
ites in the Sahara Desert reflect glacial conditions
Baltica, composed of Russia west of the Urals and resulting from this Gondwana polar location.
most of northern Europe.
The Iapetus Ocean, which had opened during the
Kazakhstania, the region between the Caspian Sea Neoproterozoic, began to narrow during the Ordovi-
on the east and China on the west. cian and Silurian. As it narrowed, a new ocean, termed
the Rheic Ocean (Fig. 10-3C), took form and gradu-
Siberia, mostly Russia east of the Urals and north of ally widened at the expense of the diminishing Iapetus
Mongolia. ocean. About the same time, an active subduction zone
formed along the eastern margin of Laurentia. Under
China, composed of China, Indochina, and the the great pressure caused by the subducting plate, the
Malay Peninsula. edge of Laurentia folded and broke into multiple
thrust faults. These crustal disturbances are referred
Gondwana, composed of South America, Africa, to as the Taconic orogeny.
India, Australia, and Antarctica.
During the Silurian, the Rheic Ocean widened to over
After Rodinia broke up about 750 million years ago, 4,000 km (2,500 miles). At the same time, the Iapetus
Laurentia drifted toward the South Pole and collided Ocean continued to narrow until its northern end com-
with what is today the west coast of Chile (Fig. 10-2). pletely closed, causing Baltica’s Caledonian orogeny.

Like its Iapetus predecessor, the Rheic Ocean was
also doomed. During the Mississippian, as Gondwana
and Laurentia converged to build the supercontinent
Gondwana, the Rheic ocean was squeezed out of
existence. In the process, North America collided
with southern Europe causing a mountain-building
event in Europe termed the Hercynian orogeny. In
the subsequent Permo-Carboniferous, western Africa
and South America were sutured to North America to
form the Alleghenian and Ouachita orogenic belts.
The building of Pangea was now accomplished.

cSOME REGIONS TRANQUIL,
OTHERS ACTIVE

The Stable Interior

FIGURE 10-2 Landmasses during the Neoproterozoic, In Chapter 8, we noted that continents can be
about 750 million years ago. Note the location of the South described in terms of cratons and orogenic belts or
Pole and Equator. In those times, most of Earth’s landmass orogens. A craton is the relatively stable part of the
was in its southern hemisphere, the opposite of today. continent consisting of a Precambrian shield and the
extension of the shield that is covered by flat-lying or
only gently deformed Phanerozoic strata.

Paleozoic strata on the craton were originally wave-
washed sands, muds, and carbonates deposited in

278 c Chapter 10. Early Paleozoic Events Kazakhstania

EQUATOR Gondwana

EQUATOR Kazakhstania

Gondwana

EQUATOR Kazàkhstania

Gondwana

FIGURE 10-3 Three configurations of Earth’s paleogeography, 514–425 million years ago.
(A) Late Cambrian, (B) Middle Ordovician, and (C) Middle Silurian. (C. R. Scotese, 2001, Atlas
of Earth History, Vol. 1, Paleogeography, PALEOMAP Project.)

Some Regions Tranquil, Others Active b 279

FIGURE 10-4 North America during the Cambrian Period. Note the paleogeographic and
tectonic elements. The Cambrian paleoequator runs almost perpendicular to today’s equator.
(NOTE: On this and other paleogeographic maps, the outlines of today’s continents and the
Great Lakes are shown for reference only. They did not exist at the time.) What were the
conditions at the location of your home during the Cambrian period? (Answers to questions appearing
within figure legends can be found in the Student Study Guide)

280 c Chapter 10. Early Paleozoic Events

FIGURE 10-5 Central platform of the United States showing major basins and domes.
(The Great Lakes are shown for reference only.) Structural section A–A0 below the map crosses

part of the Ozark dome and the coal-rich Illinois basin. These basins and domes developed at

different times during the Phanerozoic.

shallow seas that periodically flooded regions of low We recognize domes and basins by their distinctive
relief. These shallow, warm, and well-lighted seas were pattern of rock outcrops (Fig. 10-6):
favorable habitats that allowed for major diversifica-
tion of marine life. Such extensive inland seas, shown Domes—erosional truncation of domes exposes
in Figure 10-4, do not exist on Earth today. older rocks near the centers and younger rocks around
the peripheries. Sequences of strata over arches and
Here and there, sedimentary layers deposited in domes tend to be thinner. Also, because these
Paleozoic epicontinental seas are warped into broad structures were periodically above sea level, they
synclines, basins, domes, and arches, as you can see in have many erosional unconformities.
Figure 10-5. The resulting tilt to the strata is very
slight and is usually expressed in meters per kilometer Basins—compared to domes, these were more
rather than degrees (for example, strata might rise in persistently covered by inland seas, and thus
elevation by a half meter per kilometer.) In the course have fewer unconformities. They also developed
of geologic history, the arches and domes stood as low greater thickness of sedimentary rocks. In eroded
islands in the seas or as submarine banks that were basins, younger rocks are near the centers and older
barely water-covered. rocks are around the edges.

Domes and arches developed in response to verti- Orogenic Belts
cally directed forces, unlike those that formed the
compressional folds of mountain belts. Possibly, forces The North American craton is bounded on four sides
associated with plate convergence were transmitted to by orogenic belts that have been the sites of intense
the craton, causing flexures (domes or arches) in the deformation, igneous activity, and earthquakes. At
platform rocks of the craton. Where tensional forces least one of these orogenic belts is present on every
operated, basins developed. continent (Fig. 10-7). Most are located along present

Early Paleozoic Events b 281

FIGURE 10-6 Michigan basin and
Cincinnati arch. (A) In an
erosionally truncated basin such as
the Michigan basin, the youngest
beds are centrally located. (B) In a
domelike structure such as the
Cincinnati arch, the oldest beds are
located in the center.

or past margins of continents, such as the North these animals had mineralized skeletons, their very
American Cordilleran belt. small size helped to prevent their earlier discovery.

As described in Chapter 7, the passive margin of a The discovery of shelled organisms, along with the
continent may experience an early stage in which thick detection of new trace fossils, resulted in a new defini-
sequences of sediments accumulate along the conti- tion of the Neoproterozoic-Cambrian boundary.
nental shelf and rise. This stage may be followed by Today we place the boundary at the lowest (oldest)
subduction of oceanic lithosphere along continental occurrence of the feeding-burrows of the trace fossil
margins and may terminate in continent-to-continent Trichophycus (Fig. 10-8). Trichophycus and associated
collisions, as in the classic Wilson Cycle. traces reflect the first appearance of a metazoan capa-
ble of tunneling through sediment. It is early evidence
cIDENTIFYING THE BASE of bioturbation. Also, Ediacaran-type organisms are
OF THE CAMBRIAN not found above the Trichophycus boundary.

At one time, it was relatively simple to recognize the cEARLY PALEOZOIC EVENTS
boundary between the Cambrian and the underlying At the beginning of the Paleozoic Era, North America
Precambrian: The base of the Cambrian System was was relatively stable. We know little of the Paleozoic
identified by the first occurrence of shell-bearing ocean basins, for those old seafloors exceed 200 million
multicellular animals. Among these, extinct marine years in age and therefore have long since disappeared
arthropods known as trilobites were used to identify at subduction zones. However, oceans did exist—and
the Precambrian-Cambrian boundary. in places, they spilled out of their basins onto low
regions of the continents. Although a large part of the
But a problem became evident in the 1970s when a sedimentary record has been removed by erosion,
distinctive group of shelly fossils was found beneath the
lowest strata containing the first trilobites. Although

FIGURE 10-7 Cratons and
orogenic belts of North America
and Europe. An orogenic belt is the
site of one or more orogenic
(mountain-building) events. For
example, the Appalachian orogenic
belt was the site of the Taconic
orogeny during the Ordovician
Period, and the Acadian orogeny in
the Devonian Period.

282 c Chapter 10. Early Paleozoic Events

metamorphic rocks of the shield. Later, as these quartz-
producing and clay-producing land areas were reduced,
limestones and dolomites became increasingly preva-
lent. The limestones contain abundant fossils of car-
bonate-secreting marine organisms. Their distribution
indicates that shallow seas were common throughout
much of Earth’s equatorial region during the early
Paleozoic (Fig. 10-9). In fact, advances and retreats of
these epicontinental seas are the most apparent events in
the early Paleozoic history of the continental interiors.

FIGURE 10-8 The trace fossil Trichophycus. The sea cCRATONIC SEQUENCES: THE SEAS
bottom burrower mined for food by tunneling along horizontal COME IN, THE SEAS GO OUT
burrows interrupted by vertical shafts. The trace fossil is
important because the base of the Trichophycus zone is the The Paleozoic history of the North American craton is
boundary between the Neoproterozoic and the is Cambrian. marked by repeated advances (transgressions) and
(Jensen, S., The Proterozoic and Earliest Cambrian Trace retreats (regressions) of epicontinental seas. The
Fossil Record; Patterns, Problems and Perspectives, Integrative regressions exposed old seafloors to erosion, creating
and Comparative Biology: Oxford University Press.) extensive unconformities that mark the boundaries of
each transgressive-regressive cycle of deposition. We
enough remains to indicate that practically all of the also use these unconformities to correlate particular
Canadian Shield was inundated at times. sequences from one region to another.

Initially, the dominant deposits were sands and clays
derived from weathering and erosion of igneous and

FIGURE 10-9 Upper Cambrian
lithofacies map. Warm, clear,
epicontinental seas covered much of
the central United States.

Cratonic Sequences: The Seas Come in, the Seas Go Out b 283

TABL E 10- 1 Cratonic Sequences of North America

Sloss, L., 1965, Bulletin of the Geological Society of America 74:93–114.

Each sequence consists of the sediments deposited hypothesis links sea-level changes to the alternate
as the sea transgressed over an old erosional surface, buildup and melting of great ice sheets. Such events
reached its maximum inundation, and then retreated. would alternately reduce and increase the volume of
These sequences are named Sauk, Tippecanoe, water in the ocean.
Kaskaskia, and Absaroka (Table 10-1).
However, glacially linked sea-level change is not the
What caused the transgressions and regressions of only hypothesis. Many geologists think the sea-level
the seas? Because the cratonic sequences seen in changes resulted from seafloor spreading. Rapid
North America also occur on other continents, it is spreading creates high midoceanic ridges. These
likely that worldwide sea-level changes caused the ridges displace water, causing sea level to rise globally.
repeated transgressions and regressions. But what When spreading rates slowed, sea level would be
caused global sea-level changes? The favored lowered, and epicontinental seas would regress.

284 c Chapter 10. Early Paleozoic Events

cTHE SAUK AND TIPPECANOE
SEQUENCES

The First Major Transgression

During the earliest years of Sauk (Cambrian) deposi-
tion, seas were largely confined to the continental
margins (continental shelves and rises). Thus, most
of the craton was exposed and undergoing erosion. No
doubt it was a bleak and barren scene, for vascular land
plants had not yet evolved. Uninhibited by protective
plants, erosion gullied and dissected the surface of the
land. For at least 50 million years, the precambrian
crystalline rocks underwent deep weathering and must
have formed a thick, sandy “soil.” eventually, marine
waters spilled out of the marginal basins and flooded
the eroded surface of the central craton.

Islands in the Inland Sea FIGURE 10-10 The Tapeats Sandstone in vertical walls
of Deer Creek Canyon, Grand Canyon, Arizona. The
The craton was not a level, monotonous plain during Tapeats Sandstone was deposited in a nearshore
Sauk time. Instead, it had distinct upland areas com- environment of the shallow sea that transgressed a large area
posed of Precambrian igneous and metamorphic of the southwestern U.S. during the Cambrian Period.
rocks. During marine transgressions, these uplands The sandstone rests on erosionally truncated Proterozoic
became islands in early Paleozoic seas and provided rocks. It grades upward and eastward into the Bright
detrital sediments to surrounding areas. The absence Angel Shale, which in turn grades into the Muav Limestone.
of marine sediments over these upland tracts provides (N. Potter Jr./American Geological Institute)
evidence of their existence and extent. One of the
largest highlands was the Transcontinental Arch As the sea continued its eastward transgression, the
(see Fig. 10-4), which crossed the craton from Ontario early deposits of the Tapeats were covered by clays of
to Mexico. the Bright Angel Formation, and the Bright Angel was
in turn covered by the Muav Limestone. Together
By Late Cambrian time, seas extended across the these formations form a typical transgressive sequence,
southern half of the craton from Montana to New recognized by coarse deposits near the base of the
York. An apron of clean sand spread across the seafloor section and increasingly finer (and more offshore)
for many miles behind the advancing shoreline. This sediments near the top. Geologists call this a “fining
sandy facies of Cambrian deposition was replaced upward sequence.”
toward the south by carbonates (see Fig. 10-9).
Here the waters were warm, clear, and largely Cambrian rocks of the Grand Canyon region pro-
uncontaminated by clays and silts from the distant vide a glimpse of the areal variation in depositional
shield. Marine algae flourished and contributed to environments, as deduced from changing lithologic
precipitation of calcium carbonate. patterns. They also illustrate that formations usually
do not have the same age everywhere they occur. For
Cambrian Rocks in the Southwest: A example, fossils are reliable evidence that the Bright
Transgressive Succession Angel Formation is Early Cambrian in California but
mostly Middle Cambrian in the Grand Canyon. This
In the Cordilleran region, the earliest deposits were
sands, which graded westward into finer clastics and
carbonates. An excellent place to study this Sauk
transgression is in the walls of the Grand Canyon.
The Lower Cambrian Tapeats Sandstone is an initial
strand line deposit above the old Precambrian surface
(Fig. 10-10).

We can trace the Tapeats laterally and upward into
the Bright Angel Shale (Fig. 10-11). As the shoreline
shifted eastward, the depositional site for the Bright
Angel Shale was in a deeper, offshore environment
(Fig. 10-12). Next is the Muav Limestone, which
originated in a still more seaward environment.